Hey Akbar. try diluting your DNA template or reduce the number of cycles ( ex: if they are 40 cycles try 35 cycles). best regard

I agree that the second round template (first round product) should be diluted. I would use a 1/100 dilution .add 1ul to the second round primers and then run between 15 and 25 cycles of second round pcr. Remember that 10 cycles of pcr is 1000 times more product and over amplification can lead to smeared product as the pcr product reanneals with other product molecules to form long concatamers