I have a clear night retainer which fits but not entirely. My teeth have moved slightly on the top. Can it fit properly if I wear it for a while?

Probably no. Retainers are designed to be worn consistently to prevent shifting of teeth. They often need to be worn daily to keep your teeth in their ideal position. If you forget & wear it a couple of days later & it still goes to place, but is tight, then your teeth can reposition within the retainer. But, if some teeth currently do not fit, then your retainer usually will not help. See your dentist.
Retainer problem. It doesn't fit because it was not worn properly, allowing teeth/gums/bone to shift. Yes, wear retainer 24/7 until you can get back to the Dentist/Orthodontist that made it for you to check the fit and effectiveness. Retainer adjustment, or orthodontic pretreatment, may be necessary.
Perhaps. Depends on the type of retainer and how much your teeth have moved. If too much movement has occurred, you're better off having a new retainer made. It is important to be consistent with wearing a retainer as prescribed. You can try to continue wearing it, but don't force it, especially if it causes any pain. See your doctor to evaluate the fitment to see if you can keep using it. Good luck!