If you are talking about re amplifyinf already amplifies dna then you have to remember that every 10 cycles is 1000 times more material. After only a few cycles the product self anneals because there is soon more product than primer in molar terms and just forms increasingly longer smears of extending product and the band disappears. If you want to re amplify then either use nested internal primers or dilute the first round product 1/1000 and run about 10 cycles of pcr

The new DNA immediately after extraction usually retains the effectiveness of PCR.

However, if DNA is not purified from polysaccharides, as is usually the case with DNA isolation, the PCR efficiency decreases with time.

I agree with Paul. While re-amplifying, try to do dilutions. I usually do a 100X and it works fine. 

 Try to ensure PCR steps 

try to insure your temp