What charger are you using?
SLA's will suffer greatly over long term if they are not being charged properly.
A lot of ebike kits do not come with chargers that are "intelligent" enough.
They must be disconnected from the pack once the charger flips to "ok" or "full".
If you don't the charger will still put a "trickle" charge on the pack and damage them.
Additionally, every cycle down to LVC, will cause a minute amount of sulfation on the SLA plates. Every cycle compounds this. Eventually the plates are so sulfated you barely get a charge out of the pack.
Use a FULLY automatic charger WITH a continuous de-sulfation/conditioner cycle. The conditioning cycle will work to "shock" the sulfate crystals on the lead plates and encourage them to go back into solution in the battery cells.
I use a Granite Digital Save-A-Battery 36V. My 7ah 12V cells are somewhere around 5-6 years old now and and the SAB has put a few extra miles back on them for my daily commute. I have been using mine for about two years now with zero loss of capacity. I plug my packs in every night all night and carry the charger with me to charge at work. (They are very light.) The also have polarity protection so you won't blow it up if you get your wiring backwards on the packs accidentally.
Save A Battery 2365-36 36-Volt Battery Charger and Maintainer via Amazon @ $85 or from their home page for $100.
There's a particular model of Soneil chargers that can do this as well. They cost about 50% more and I haven't tried those. "SR" models... I think.
-DS