Plugin Updates not showing

Hi folks,
I’ve had this issue on my site for quite a few months now. And it has persisted through numerous WP versions. I now have 3.6 and it’s still occurring.

Basically WP is not showing me available plugin updates.
It might show it when I first log into WP admin (it will show a number next to the Plugins menu item) but as when I go to the plugins page it shows no updates.

How would I resolve something like this?

Thanks.