Voltage is one thing. It is like water pressure.
Amperage is like the flow rate of water at any given pressure.
A squirt gun has lots of water pressure, but no amperage (or flow rate) behind it.
A fire hose has both ..... (knock you over and roll you right along, it will)
You are telling us your bathtub is acting empty but your squirt gun seems to be working well --- but you can't seem to get your tub filled up all the way full for very much.
A normal motorcycle battery charger is a 1-2 amp charger (a garden hose flow) which will fill your battery up in about 8-14 hours from totally empty. A properly charged battery (fully charged) yields a lasting, stable voltage of at least 12.54 volts
which hangs in there after a shot or two of cranking load.
Check the output on your trickle charger. Sure, it operates at full voltage alright, but I bet it has an output that is measured in milliamps, not amps.