* Use more template. Sample concentration may be too low. * Template DNA may be damaged. Use carefully purified template. * Optimize enzyme concentration by testing a titration of enzyme in the reaction (0.25-2 units/50μl reactions)* Increase number of cycles. * If HF buffer has failed, try using GC buffer. GC buffer allows better amplification of GC rich and longer templates, but slightly lowers fidelity. * Lengthen extension time.