FAQ: Why are there high molecular weight smears or DNA in the wells of an agarose gel after a PCR using Phusion® High-Fidelity DNA Polymerase?

Strong protein binding to the DNA may be preventing proper gel separation. Try adding 1% SDS to the loading dye just before running the gel.
Or non-specific products are being amplified. To avoid this we suggest the following:
* Reduce polymerase concentration.
* Shorten extension time.
* Reduce total number of cycles.
* Increase annealing temperature or try 2-step protocol.
* Optimize Mg2+ concentration.
* Lower primer concentration.