20 cycles of pcr can be as much as 1000000 times more pcr product so it is really easy to over amplify your template to the point where product molecules anneal to each other and form longer concatamers which will look like a smear of pcr product. Set up some pcr second round amplification tubes and remove tubes from the pcr machine at 18, 18. 20......26 cycles and see where you get a clean product. As template you could use 1:100 dilution of the first round product