Now then mate.It does sound like you have a mental block in throwing the uppercuts.It sounds like it could mostly be your conscious factor that the object your used to throwing uppercuts at is no longer a strung up punching bag that waits for you to line up and unload your punches at your own beckon time.The fact that the object your finding differcult to throw uppercuts at is a formidable moving live object(Sparring Partner)that is entitled to hit you back at any giving time is the problem.Dont be afraid to get up close and personal and unleash your great uppercuts on the sparring partner.After all thats the whole purpose on sparring,to sharpen up your punches!Just let your punches roll off without no hesitation and have your trainer watch you at close view as you are doing soo.That way you'll always have a good feedback on how your punches roll off.The key is,just try and get used to throwing them without much hesitation at all.Good luck and enjoy your sparring session