Usually velocity sensitivity is done by having 2 switches under the key, one gets triggered before the key is all the way down, and the second gets triggered at the very bottom of the key press.
The keyboard measures the difference in time between the triggering of these two switches and uses that to get velocity data.
What is probably going on is that one of these switches is either dirty, stuck or not working... or some mechanical issue is getting in the way.
If you know your way around electronics, you can open the keyboard and try to clean the switch with rubbing alcohol or electronics cleaner. If there is a mechanical problem, like broken plastic on a key, you can try to repair or replace the part.
If you have an ohm meter, check the continuity of the switches, if there is a problem and cleaning doesn’t fix it, replace the switch. If it is a rubber button switch, forget about ohm meter, just clean the black contact on the rubber with some rubbing alcohol and a toothbrush, that usually does the trick.
Your keyboard might have a different method of getting velocity data, but the above is the most common I think.
It shouldn’t be too hard to fix, but expect to be charged a lot by a shop…
