Now will only work on main site, wont work on MU sites

Having issue with this plug in was all working fine for both main site and MU sites.

Now will only work on main site, wont work on MU sites.

If you subscribe on main site all works fine, email is added, yet if same thing is done on sub site it comes up as if its added but when you look at subscription no new email comes up on MU site subscriptions page or main site ?

I have uninstalled and re installed but same issue, look like it has done this since 3.5 upgrade.?

So not sure what the issue is?