I have had similar problems in trying to digest PCR products (its even
worse with 2 enzymes) and directly cloning. To get around this, I
subclone my PCR products (without digesting) into the TA vector. I
then can grow cut it out of this vector by digesting with the
restriction enzymes that are on your primers and subclone as usual.
This also eliminates the problem of not knowing if your PCR product has
actually been cut with the restriction enzymes. It a little bit more
work, but believe me, it saves time in the long run.