Our home was built in 2001 with new construction tilt-in vinyl windows and several of the windows are hard to raise… I didn't see any screws to adjust inside window sash (since they were probably nailed around the flap during construction). I also have 2 windows that once opened, don't seem to want to stay in place and slide down a little…..
Rural Retreat, VA
I'm sorry to say, it sounds like your windows might have been very poorly installed.
Windows that are difficult to move are usually out of square, and ones that slide could have weak or broken springs.
This might be a situation where you have to tolerate the hassle until you're ready to replace the windows altogether. The good news is that energy efficient windows have jumped lightyears since 2001, so new ones installed today or down the road will earn you a solid return on investment.