We just covered this problem at our quilt meeting last night.
when you fold your fabric selvage to selvage are you trying to match the cut edge of the fabric??????? Well you shouldn't as it may not have been cut straight at the fabric store.........
Make sure the selvage edges match and that there are no bows and that the fabric hangs straight........ don't worry if the cut edge doesn't match.
even the cut edge............ then cut your strip....... works every time.
Hope it works for you.
Scrap Quilter