This symptom may appear if the missing LPARs were cloned from other LPARs.

According to IBM:

An LPAR cloned from a mksysb may still have the RMC config from the mksysb source. In this case, you may see IBM.DRM active but if it does not return after running the recfgct and rmcctrl commands listed above, then it was never really active.

To confirm if this is the case, IBM suggests to run the lspartition -dlpar command on the HMC and if the LPAR does not show up then run the following commands:

/usr/sbin/rsct/bin/rmcctrl -p