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.