It sounds like the sinew is lifting ,probally not making a good bond to the bow back. Is it a straight bow? Or is the back recurved or reflexed ,and the sinew is lifting in one of these curves? Did you degrease the back with acetone or some other degreaser? And did you rough the back up a bit, and 'size' the back with a couple of coats of thin glue over a few days before you started sinewing?
If it's an otherwise good bond and it's just lifting near a recurve or other bend in the back of the bow ,get some glue under the lift (I use a big syringe)and wrap the whole bow with an ace bandage .Making the wrap tight enough to pull the sinew back down ,and squizze excess glue though the bandage. Then I would leave it alone for a day or two,and then rewrap the bandage in the other direction.
Ralph