Unable to install addon for appointments lite

Hi all,

I'm using Appointments lite and am trying to install the 'additional fields' add-on, however every time I select it and try to activate it the screen refreshes but it's still not active. There is no message or anything to point out what the problem is.

I've added a couple of add-ons for this plugin previously but now can't seem to enable any others.

I saw another post in the forum where it was suggested to uninstall lite and install + but lite is the only option available...

Any ideas?

Many thanks in advance,