Hi Wenhao, concentration of your DNA is quite lower in the sample but you can improve your PCR product’s concentration. Here are a few recommendations:
First thing you need to do is that your PCR reaction should be well optimized (almost in every aspect if possible).
Run a nested PCR to increase the concentration of DNA in the product; that’d be what’s referred to as ‘PCR of the products of your first PCR’.
Check your all of the components of the mastermix. Use high quality reagents.
Inappropriate primers can affect your results. So design appropriate primers.
Well optimization of annealing temperature is also required for improving the yield. You should adjust it in a 1-2°C step. Adjust the duration and temperature of denaturation, annealing, and extension efficiently as it will greatly impinge on your PCR product.
Check your sample for contaminants. They will inhibit the PCR. Try removing them in a way that’d be suitable for you.
Low conc is not a problem if your yield is high enough. If none of the optimization steps work you can use multiple PCRs and pool the products together, then purify with less elution buffer.