If your sliding windows are hard to open, you probably have gunk built up in the sliding track. The good news is this can be cleaned. Remove the sash from the window frame and clean the track with a simple mix of soap and water. When you’re done, you might want to lubricate the rollers on the window using a non-silicone lubricant.
Sliding windows that stick all the time may have bad rollers. It’s possible to replace the rollers, but if the rollers are failing, it could be due to age and it might be worth replacing old sliding windows.