Cant finish fence

[Hecate]

Hi guys this is my problem, im building a fence and have come to the last piece to build and it wont let me place it, so i have a gap in my fence that i cant build in does any know why or how to get round this ? thx

[Emina]

you are probably gonna have show us a small screenchot of the small area you are having problems with, for anyone to be able to tell what went wrong.