have you had a 12 week scan? did they mention where the placenta was? if its at the front this cushions kicks meaning that some women dont feel them until mid 20 weeks.
also baby has a lot of room in there to wriggle around and as such may be kicking your back which will also make it harder to feel.
Plus in the early weeks, unless your sitting down quietly and really concentrating, it can be hard to pick up on those little poppy feeling kicks.
I hope you feel it soon, in the meantime if you do see midwife she may well try and find heartbeat (its difficult at 17 weeks) just to put you at ease.