If a transaction has been previously allocated, then it's allocation history will not allow you to allocate this again to any period prior. This is a default setting, however this can be disabled to allow you to allocate to any open period.
This setting can be disabled by doing the following:
Click System, Application Configuration, System Configuration
On the General tab, untick Always allocate cash to the latest year/period of the selected transactions.
We would recommend that this option is then re-ticked after allocating the transaction.
However, if you do the above and you are still unable to allocate, then get this passed through to support include details of the transaction affected.