high internal resistance brings battery voltage back

if you are careful one can open the vent and add a bit of water to each cell.otherwise recycle it.
if you got it used it has likely been pulled due to age or failure.
i repair lots of stuff using these and sometimes you get an oddball early life failure in 1 battery out of 4,8,ect but replace the group to keep balance in the series string.the others get load tested and go on the hamfest table for the qrp guys.
i have revived a few small AGM battery but its generally a losing proposition.i wont even bother now.
the larger ones if not over 5 years old respond well to rehydration.just did 4 8d out of a telecom site that were floated too high for about 6 months.they would not pass a run test and were replaced.they flunked a 50a test from a cheap "cheese grater" tester.after addition of 8oz water in each cell they woke up.passed even a 500a test with my big carbon pile.
" proper terminal voltage " has no meaning without numbers , the statement implies 12.75 - no load .
Load testing of a battery is done with a load-tester [ carbon pile or large resistor wire ] & voltmeter , then also specific gravity of electrolyte in each cell .
A lot of battery maintenance / repair came from early batteries , the ones that the cells were separated by tar on top of the case , then you could remove the tar and load-test each cell to determine which one was bad , then desolder and replace .
I worked with an old navy radio tech , mostly dropped off at sea to submarines and when I worked with him , he had a used truck & equipment bis. and we would do the same with modern plastic case batteries , the only difference was using a hot-knife to cut open the case to test & repair , then seal back together with hot-glue gun .
Another repair was getting rid of the sediment at the the bottom of the battery , as the batteries go through the charge / discharge cycling , the plates sluff off particals that sink to the bottom of the cell , eventually rising to the bottom of the plates and shorting them out .
The fix there was to [ modern cases ] drill a hole in bottom of each cell to drain electrolyte and flush with distilled water till clear , then hot-glue add new electrolyte .
The only failure we did not fix was when the plates buckled , except to just replace .
Now weather you want to go through all that is the question , they are small & cheap so no loss for the learning .
These things go bad if they're not used, and they seem to go bad after a short number of years, anyway. If it is a couple of years or more old, or it has been sitting for a while without a charge, it could be bad. But we don't know, not enough info, given the OP. It should charge at about 14 volts for many hours, then test with a load. My guess is that a good power station battery with a one amp load should maintain 12+ volts for several hours.