Maybe You can see a 4705 error or somewhat in your squid logs?
I just had a problem where Guncraft would tell me that it could not sync with steam and could only play in offline mode. But wait a minute… my friends can play! Sure enough a weird line in the proxy logs! This is a Squid3 issue but we can fix it.
Fix:
Add the line:
ignore_expect_100 on |
to your /etc/squid3/squid.conf file
Other apps that can have this problem:
AppBrain
Facebook Contact Sync
BOINC