Hi
Sorry not been into electronics for quite awhile so a bit slow !.
Have a Ryobi 4v cordless drill, problem not charging, voltage at input 6.2v, voltage to battery 2.28v !, what do I have to check in between ?.
It is also possible that the charge rate is very low because the battery is so low, and that leaving it on charge will get some charge into the battery, at which point it will charge faster.
If it's a lithium cell and the voltage on it has been below around 3V for more than a brief time, it may be developing internal shorts and will not then recharge.
The charge controller IC will only pass a very small current to test the cell, until it reaches its normal working range, as Diver mentions.
Trying to fast charge or force charge one that has started to fail can make it overheat or burst, hence the low current testing stage.