Trouble creating large files for banners

I've been making banners for our local high school for years and for some reason just recently I'm having trouble uploading them into the lab that I have used for many  years.  I'm not doing anything different than I did before which is really frustrating because I can't figure out what is going on.  My banners are 32x44 inches and are at 300 dpi.  I start with a plain document, pull my image into place and adjust to fit, add all wording and then flatten the image. Everything that I've always done, but the lab says image unavailable when I can clearly see it in the image table.  The lab says it's not anything to do with their software so it must be an issue on my end.  Has anyone had similar issues?