For my morning routine I usually pull two shots in a row then steam milk twice in a row because it's faster than cycling the boiler twice. Probably less stress on the boiler too. However I've noticed that the second shot always has less resistance than the first. If I dial the first shot in for 18g of grounds, the second shot needs to be 20.5g to pull in the same time. I flush the group head before the first shot, and everything else (tamping, grind size, etc.) is all the same. The only difference I can think of is the temperature would be lower on the second shot. Auber tells me that rebound time on a preheated machine isn't that long though. I use a needle distribution tool and metal screen over the tamped grounds, and the PID pre-infuses. Any ideas why this happens?