Tonguing on a trombone?

I play trombone and I can double tongue to a certain extent. Sometimes I can do it really fast but my tongue occasionally "sticks" to my teeth and I stop. For some reason I can play really fast while single tonguing some times. I always find it really hard to tongue (single) the very first note of a song or new entry but I can do the rest. Usually I just blow air into the horn for those notes. What am I doing wrong? Does the fact that I have a slight lisp make a difference?