Transaction Locking & PO Conversion

Transaction Locking & PO Conversion

We have encountered a problem regarding transaction locking and trying to convert a PO to a bill. Even if the PO we are trying to convert is dated in a non-locked period AND the bill date is also in a non-locked period, we cannot convert the PO if items on that have transactions linked to them in a locked period. We have been told this is due to FIFO (which is not accurate). If we have to unlock each period prior to converting a PO to a bill, what is the point of transaction locking? This is not how other softwares work. Is anyone else having this problem?