Thanks for your patience for a reply while we were away!
Hm, there was one version a few versions ago which had a little one-off glitch that said that your version was up to date, however there was actually a new update available. Are you on version 21.7.0? You can check up the top right hand side of the splashscreen as per the attached screenshot.
If you aren't on this version, you can update here: https://plusspec.com/release-notes/
If you find an update isn't the problem, just flick me a quick email and we'll do some further investigating.