Diagnostics of website failure for only some users

Hello,

I run a regular Apache + mod_wsgi + nginx website on my Linode and encountered a strange problem. The website works fine from my computer and also from our managers' one (who use a different Internet provider). However, some of our website users report to managers that they can't open it in their browser, for a long time period, while at the same moment it worked from my computer and managers'.

The question is: how should I do the diagnostics (if this bug is even my fault) and what bug should I look for if it is impossible for me (even if I wanted to) to go to the problematic user and do the diagnostics from their computer? I appreciate if you could help :)