First, does this only happen on school computers or through a school account? Or does it happen at home on a personal computer as well?
Second, try this: create a “test” test, made up of problems with NO equations and NO graphics. Search through and find a few of those types of problems (remember: no equations and no graphics) and save the “test.” Try downloading that one. If it downloads, that indicates that your site has a “timeout” security measure. These “timeout” walls stop the downloading when it perceives it as taking too long. With equations and graphics, it can take longer to download. If you can download the test that has no equations nor graphics, you will need to talk to your IT department about making the “timeout” longer to allow you to download your tests. Or, you can download at home.