Can something cause infertility after 3 kids

Kenzie • 3 wonderful boys. Trying to get our 4 th baby!!! Maybe a girl ???
I have 3 fabulous little boys. To whom I conceived the first month with all 3. I know I know. Maybe it was luck.  We are now trying for number 4 and final. And going to month 5 nothing :(.  Any advice.   My youngest son is 2.  Would something suddenly change in that short of time ??