I encountered something like that too when playing a straight warlock build recently. When I hit lvl 16 I could only select greater invocations, no dark. On level 18 though I got dark invocations. A guess from me, by experience with the PnP-D&D-3/3.5ed.-Tool eTools: maybe the warlock invocations were erroneousley bound to the charisma-score the way spells are bound to intelligence/charisma/wisdom?
Edit: Of course I mean the" highest learnable level of spells = base ability -10" rule. Not the DC or something...