Does the charger shut off/ LED turn green at the end of charge in a normal amount of time?
If so you're probably reaching proper full charge voltage. Also your display should tell you if you have a full charge.
Typically a battery issue will show itself with premature shutdown.
I think the smaller battery is the difference from your other bike. Larger batteries are just more stable and efficient at the cost of added weight.
On the same bike I use a 10ah and a 21ah battery. The 21ah has a noticeable more than double the range and far less voltage drop under load.