The titer might be low. We use a p24 ELISA kit to measure the titer: https://www.origene.com/products/cdna-clones/lentiviral-particles/lenti-titer-p24-elisa-kit
If the titer is low, you can try Lentiviral Concentrator: https://www.origene.com/products/cdna-clones/lentiviral-particles/lenti-concentrator