Hey Lisa,
Make sure your blade holder is seated all the way down in the carriage
and make sure the thumb screw is tight.
I had the same issue and found the pressure off the cutter was pushing
my blade holder up as it was cutting. It would start out cutting fine
until about three quarters of the cut it would barely cut through the vinyl.
I made sure the blade holder was seated all the way down, and that it was tight
in the carriage and now it cut's fine.
Hope this helps!