This one crops up repeatedly so I thought that I'd do a FAQ on it...
The most common reason is that you have a current membership in place, it has not yet expired and you have used up some or all membership bundled allowances and so cannot use the relevant services.
When you have a current membership in place and buy a new one the new one does not start immediately. It "stacks" to kick in when the current one eventually runs its full monthly course and expires. You do not get access to the new membership's bundled allowances until then.
Note that you are charged for the new membership at the time of purchase but you have not "lost" the money paid as the membership has stacked and will start once the current membership expires.
Memberships always run their full monthly course and cannot be terminated early. In particular buying a new membership does not terminate a current one.
If you use up current membership allowances then you really need to buy add-ons rather than a new membership in order to be able to use the relevant services again under the current membership:
When there is any confusion about this you can usually clarify matters by checking your current membership status and expiry date on the My 48 page of your online account:
The right hand side of this page also contains the Membership Planner section which will show if there is a stacked membership waiting to kick in on the specified date (immediately after the current membership expires).
You can also check the My History page to check recent payments/purchases:
Hope this helps.