Hi Ferry!
The first thing I want to highlight is that you should not modify core plugin code – please instead refer to our Themer’s Guide which covers the correct (and safest) way to override and customize our views (of course, if you are already doing that – great!).
To the problem at hand, I’m afraid I am unfamiliar with BFI Thumb. If the problem only occurs on ajax requests, though, perhaps the issue is that it relies on a browser event – such as document.ready or window.onload – to trigger its particular behaviours and that only happens once per page load.
Perhaps you could contact the author (of BFI Thumb) to ask if that might be the case and, if so, check if there is some other way of triggering it at a later stage.