I think it's normal for the TH to diminish after a few hits since the throat gets used to vapor. I usually like to take 4-5 solid hits, lay the PV down for a few minutes and repeat. When I used to chain-vape, I only got great TH in the morning or after I ate.
I also find that I don't get a good TH when I'm really active. When I'm relaxed in front of the computer or in my office everything is peachy. But if I'm running around like a chicken without a head, I really need to up my nic levels to get great TH. It helps to switch the juice often throughout the day as well.