Overtime dirt and grime will build up inside the window frame and stick to the window shoes which are connected to the sash. The shoes are part of the balance system which makes both sashes on your double hung move up and down. Spray down the shoes really well with a silicone sealant and operate both sashes a few times to disperse the lubricant throughout the balance system. Only use a silicone based lubricant, other types will collect dirt which will gunk up the shoes even worse overtime.

On a sliding window, remove both sashes from the frame a clean up the rollers on the bottom of the sashes and remove the dirt from the bottom tracks and weep holes. A clean track and weep holes provide maximum performance and drainage, the last thing you want is water sitting within the frame and potential damaging the structure of the opening. When everything is all cleaned up, spray a silicone lubricant on the tracks where the sashes sit, this will allow easy functioning and will prevent the rollers from sticking. 

When it is that time to clean window glass make sure you don’t have any broken seals. A broken seal occurs when air the two panes of glass separate from the spacer and create a foggy appearance you cannot clean. Whenever a broken seal occurs, the argon gas escaped the glass, and besides being horrible to look through your window glass is now acceptable to moisture and air leakage which reduces energy efficiency. To solve this issue, contact the company that installed the product or call the manufacturer, if under warranty they will replace the glass.