I recently began using the pTRIPZ system. I added my specific shRNA to knock down my gene of interest, and then packaged the vector into a lentivirus for transduction. After transduction, I performed selection using puromycin. And then I added different concentrations of doxycycline, ranging from 1ug/mL to 5ug/mL. I am seeing a large amount of RFP expression. I harvested cells at 24, 48, and 72h. And for time points beyond 24h, I added fresh media with doxycycline. But after performing western blot, I do not see a decrease in my protein of interest. What could be the problem?