What is size of your DNA fragment?? What is AT/GC %??

Some times size and GC % affect such reactions.

Have you check with repeat template preparation?

Phusion has way higher processivity and lower elongation time than Taq, so it is overall a more efficient enzyme.

It happens. size of the DNA fragment to be amplified matters a lot. Phusion is best choice for amplification of large DNA fragments

If you are cloning restriction sites onto the ends of your cloned gene then you can't use Taq as it has 5'-3' exonuclease activity, which will chew away the 5' ends of primer overhangs