This is very common with Pella. I explain to the customer what they (the windows) might do before we clean the first window. Pella frames are hollow so when the "ports" on the side get clogged with insects, dirt, etc. they will fog up quick.
It is a good idea to clean those ports out regardless of this issue. Leaving them clogged up will cause the window to get hazy quicker in between. When I do up a whole house in and out and in between, I shouldnt have to do them in between again for 4-5 years. I get quite a few customers that are upset with Pella windows due to moisture buildup in between the panes (usually in the morning).
I was told by the Pella service manager in my area that this condition is due to the paint or stain that is between the insert(AKA storm window) and the main window. Worn, dried out paint or stain can cause the wood around the window to retain moisture. You will notice this more so in the morning or late afternoon.