Is the base game also from PKGj, or was it installed via another method (ie Maidump/VPK), or is it a legit game?
From what I recall, if it's from Maidump/a VPK, then you can't use NoNpDRM stuff on them, since VPKs/Maidump are decrypted games and NoNPDRM is encrypted.
I believe with legit games it can be hit or miss, depending on the game. Some legit games might not see pirated DLC because of a license conflict (ie pirated games have different/no license, whereas your legit game does) so it may not load the DLC. If it's legit, try using PKGj to install the game and see if that helps.
If it's also from PKGj, when you finish installing the DLC, try refreshing the live area (which can be done in VitaShell). Normally PKGj will do that for you, but I'm not 100% sure if it still does for DLCs or not (haven't installed any myself).