discovery dry cell/ magnum inverter questions.

Regarding your low battery... I looked on the Discovery website and did not find any information about any "special" methods of trying to "recover" an AGM type battery. The only suggestion I can make is to take the battery out and charge it with a 6 volt battery charger and see if it can be brought back up to full/equal voltage with the rest.

Another suggestion would be to charge your bank at the "absorb" voltage set-point (14.4 VDC or whatever your batteries require) and hold it for 8-24 hours. With flooded cell batteries, they "leak enough energy" (by "slightly overcharging" the other series cells) to "bring up" the low cells/batteries. With AGM, they "leak" very little extra current through them so they do bring up the "weak cells" as easily. Also, AGMs (in general) do not like to be over charged (there are exceptions, but a quick search of the Discover library did not have any information about what to do or not).

If you do not have a 6 volt charger, you could try placing something like 6 volt brake light lamps across each other 6 volt battery in the string--The brake lamp(s) will bypass some current around the "good batteries" and "trickle charge" the one low battery and bring up its voltage (if it still good). Just monitor the weak battery once every hour or so with a DMM so you can disconnect the lamp(s) across the other battery(ies) when it is fully charged.

A Sylvania 6 volt 1129 bulb will pass around 2.63 amps. If your 390 AH 6 volt battery is 10% low, then (390AH*0.1) 39 AH (39AH/2.63A) would take ~14.8 hours of charging to bring up 10% (just to give you an idea of the time frame).

Note there are good and bad ways to connect parallel battery strings--This website does a good job of explaining:

http://www.smartgauge.co.uk/batt_con.html

Regarding the AC inverter issues... Have you measured the DC Voltage at the inverter's input terminals... Inverters typically have some operating input voltage range... Typically 10.5 to 15.x volts (12 volt battery bank) or 21.0 to 30.x volts for a 24 volt bank, etc... Other than checking the battery bank voltage and current values, there is usually nothing user serviceable inside an AC Inverter (sometimes there are input fuses inside inverters--But that does not sound like your issue).

Checking cables, cable connections, etc. is important too. Any dirty/loose/corroded connection (or cable wiring) will increase the voltage drop and dramatically reduce the surge capability of your AC inverter (high starting surge on AC, causes high current on DC side, trips the low battery cutoff voltage).

A quick way to check connections is to set your volt meter to 2.000 volt full scale, And while there is "relatively" high current flow through your battery bank cables (during charging or during discharging), take the meter and measure the voltage drop across each cable and connection. (i.e. from cable copper inside crimp to battery lead lug)... Each of these measurements should be well below 0.01 volts. Any "high readings" could indicate a high resistance connection.

You  can also (carefully) put your hand on each connection/cable (after many minutes of current flow) to see if any are warm/hot to the touch.

If you don't have one, an AC+DC Current Camp DMM is very handy to have. For example, with your multiple parallel battery strings you can measure the current flow through each string (charging, discharging) and see that they are all properly sharing current or if there are low or high current strings (look for bad connections, weak batteries, etc.). Current Clamp meters are very easy and safe to use... Just set the meter and "clip" around one cable to measure the current. There are AC only clamp meters, but they will not work for measuring DC current flow. A couple examples of current clamp meters:

https://www.amazon.com/UNI-T-Digital-Handheld-Resistance-Capacitance/dp/B0188WD1NE (low cost, good enough)
https://www.amazon.com/Auto-Ranging-Resistance-Klein-Tools-CL800/dp/B019CY4FB4 (mid priced meter)

When working on battery banks... You must be extremely careful. These battery banks can output 100's to 1,000's of Amperes and create "arc flashes" that can seriously injure or kill. Also Lead Acid/AGM batteries create hydrogen gas during charging. Wearing a face shield and cotton cloths that you don't care about (electrolyte/acid ruining cloth) helps. Remove rings, gold chains, etc. when working on electrical circuits too.

Another common practice is to wrap the exposed metal on wrenches and screwdrivers with electrician's tape to prevent short circuits across battery circuits.

Just be very careful. You cannot "turn off" a battery bank.

Regarding pumping with a second low power pump from (in well?) pump. A surface pump can only "lift" water 20-25 feet, and you may not be able to draw water through the first pump.

Pretty much everything here--The details matter. Brands/models//configuration/pump types/etc... Electrical devices will eventually fail--For power electronics, 5-10 years is when failures can occur--And 10+ Years is a "good life" (yours may last longer or shorter times). For repairs, over 5 years, it can be difficult to get manufacturer's support, and over 5-10+ years many electrical parts are difficult to find.

-Bill