Use a high-fidelity polymerase (Phusion, Platinum Superfi II, KAPA long range, TaKaRa PrimeSTAR GXL, etc) and/or use longer extension time

even though the amount of cDNA is small PCR is hugely amplifying so if the next step is pcr of the cDNA then PCR or nested Pcr should find enough template to amplify

Péter Gyarmati Thank you so much for your advice, I will try different PCR enzymes and extension time.

Paul Rutland Yes, I think the template is definitely enough, but the product is much less than I expect. That's why I suspect something is wrong in my PCR solutions and PCR procedures.

by using PCR technique called gap PCR