I have the popular Shimano, Nexus 7 speed, internal gear hub. After removing the back wheel to replace the old tire, I've found that changing gears especially from 5 to 6 and 6 to 7 (especially the latter) is very tight.

It's however still possible, and all the gear shifting seems to work as it should, except the very tight pull required in the higher gears. The gears are adjusted (the red dots point at each other perfectly in gear 4).

Any ideas what could be causing this, and/or what I could do to fix it back to normal?