why am I not getting pregnant this time?!?!

Sorry just needed to vent! I'm getting so frustrated with all of the BFN's .... i don't know why I'm having such a hard time conceiving this time around.. I already have 2 children whom I conceived very easily; I even got pregnant while on birth control but that ended in an ectopic... and now when I'm trying for #3 Nothing! I just do not know what is going on!