Pump users: anyone experience a site warm-up time?

I've been pumping over 10 years, but recently I've noticed that when I start a new pump site, it take 4-6 hours before it seems the insulin is really working. This confuses me because I don't have a lot of scar tissue, and the site DOES start working after several hours (so it isn't a crimped infuser) . Was curious if anyone else has experienced this? Does anyone seem to have a high insulin resistance at new pump sites that eventually goes away? Another thing I've wondered about is my resistance to Humalog. I use Humalog exclusively but at times it seems to take a lot longer to have an effect on my blood sugars.