It's possible that you are doing the technique wrong. Find the problem and fix it.
It's possible that they are being obnoxious and stopping you just to prove they can. In my dojo, the appropriate response would be to do something else, rather than attempting to force the specified technique when it isn't actually there. But training conventions differ -- ask a senior student at your dojo.
If they are injured or in pain, stop. Aikido does not depend on pain compliance. Figure out a non-painful/injurious way to do the technique.
I second this.
You may also wish to look for discussions on issues such as"good uke" "bad uke" and similar - the possible responses vary