There could be multiple reasons for writer's block.

1) You may just not be in the mood to write. Never force yourself. Let it come to you and when it does start writing.

2) Not enough inspiration. Maybe you are stuck on a word to use or not sure where to go with a particular part.

3) Having doubts about your story? Maybe you are having second thoughts if your story is even worth the time. Take time to brain storm on new ideas to make your story different.

4) Maybe it is that you just can't concentrate. Put away all distractions. (including your phone or ipad).