Last fall I had a vinyl slider window installed in a 5 foot wide by 3 foot high space. Both panes are difficult to open and get "stuck" about halfway and to close them, I have to put all my weight into the pull. I waxed the top and bottom of the frame but that made little improvement. Unfortunately, the installer closed shop so I'm on my own.
Answer:
It sounds like the 5-ft. span is too great for the frame of the window. You may have to do one of two things - either install (as inconspicuously as possible) a support post in the middle or up-grade the lintel above the window.
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