Are you saying you have to sneeze but can't, or you do but your sinuses are so plugged up it's difficult to actually sneeze??
If it's the first one, and you just have to sneeze but can't, try:
1. Look into a light. It sounds strange but it works for me. If i see a bright light, it makes me wanna sneeze. Maybe that's just me though.
2. Pluck your eyebrows (if you're willing.) When you pluck your eyebrows, it makes your nose run and sometimes will make you have to sneeze.
Hopefully I helped. If not, oh well, I'm tired, too.
Bob: "there is a chance, albeit a microscopic one, that our baby at some point in his or her formative years will get lodged in a tree"
~Related~