My guess is that you had already manually activated the 'Approach Phase' on the PERF page, prior trying to insert the hold. In this case, if the missed-approach involves that hold, you will not be able to insert it again.
However, had you remained in 'Descent Phase' - No probs.
You can 'fix' this by inserting a new cruise level (CRZ) on the PROG page, selecting it on the FCU, pull ALT briefly, then continue descending in OPEN or V/S. this will re-activate 'Descent Phase'.
Whether it's appropriate to be 'heads down' to achieve this depends on the time/distance to run, and the environment (busy TMA?) but it is a handy trick to remember. It's also useful if after a go-around, GA fails to engage,then you are once again stuck in 'Approach'.