This is because the matter relating to the case is not open so the monies allocate against the unallocated matter.
You will need to check the case and make sure all the relevant information is filled in correctly, for example the Client Ref.
Once this has been checked and the linked actions run to create the matter you will need to reverse the posting.
This can be done as below:
Put the unallocated matter in when selecting the posting type and ticking Reversal in General Posting.
Select the posting that has gone against the matter to reverse in the allocation window.
Select Save with the Floppy Disk icon and then POST.
You can then post it back onto the correct matter that has been created through General Postings.