Why is it sometimes difficult to resuspend E. coli in P1?
This is particularly an issue when you do maxipreps and the pellet is 200x the size of the pellet from a miniprep. The reasons why? Maybe you're pelleting at too high of a g-force (pure speculation). Alternatively, you could be growing the cells too long. I typically miniprep after a 12 hr culture. If you're seeing any darker colors in your pellet, you have a significant amount of dead cells. Those dead lysed cells will spill out their genomic DNA which is very viscous.

For maxipreps, you can't break up the pellet merely by pipetting up and down a few times and calling it a day. What you can do is swirl the side where the pellet is and breakup the clumps. Alternatively you can use a different pipet tip (for an extensive discussion on this very topic see Pipetting damage to cells). I change to a P200 for my minipreps and pipetting only the area around the pellet seems to do the trick for me. For maxipreps I use a P1000 which is better than using a 5mL syringe.