Repository Plugins not showing available updates

I have several Plugins installed from the www.ads-software.com repository, and none of them are showing available updates, even when updates are definitely available.

A couple of paid third party plugins show their updates, but the repository plugins don’t show as available.

For instance, I recently saw a notice from a Modern Tribe paid plugin (The Events Calendar Pro) that an upgrade was available. I could install that automatic update just fine, but the main, free, www.ads-software.com repository plugin (The Events Calendar) also needed updated, but my installed version (3.4) said it was the most up to date version. Searching the repository through Plugins > Add New shows the most recent version as 3.5. It is impossible to upgrade plugins through the Multisite Admin, which makes me wonder how many other plugins have missed critical updates.

Any ideas?