On my site – Link removed at members request- I'm having problems with

On my site Link removed at members request I’m having problems with user registration.

Free users register at Link removed at members request and they are then sent an activation email with a link to click on. They click on this and get told;

‘Activate your Account

Please provide a valid activation key.’

Then they go back to their email, and if they get this part (which I can’t expect jo bloggs to get, ie my mum would never work this out!) they then have to copy and paste the activation key which comes within their link ie –

Thanks for registering! To complete the activation of your account please click the following link:

Link removed at members request

Then they go back to the page asking them for a valid activation key, insert the key and the page simply swallows up the key and refreshes to make a blank box again, prompting them once again to enter a valid activation key.

It basically looks like it hasn’t worked.

Then you go to try to log in as the new user and you actually can log in now! (again my mum wouldnt even think to try this, she’d have given up by now).

How can I fix this please?

Thanks

Sally