If you are experiencing low yield of PCR product, then try the following to improve output:

• If the concentration of your DNA sample is too low, then use ethanol precipitation to concentrate your DNA sample
• Increase the number of PCR cycles to 45
• Optimize your annealing temperature
• Optimize your primer concentration, the concentration of each primer should range between 0.1 to 0.5 µM, but for most PCR applications 0.2 µM is sufficient
• Optimize your dNTPs concentration, each dNTPs should have a concentration of 200 µM
• Check primer design