By it not fitting do you mean it doesn't sit flush with the back assembly? Make sure you don't have any stray screws left inside the iPhone. Sometimes they can get stuck to the adhesive on the LCD backplate or inside small gaps in the logic board.

Also depending on who you got the Digitiser Assembly from, sometimes they are manufactured poorly and the glass has not been moulded correctly. If there are no screws stopping the digitiser from sitting and it is the digitiser itself, take a picture of the fault and send it to the manufacturer to organise a refund or replacement part.

Hope this helps :)