This is a known issue, but I don't know if it's on Runey's priority list of corrections. I personally believe that the flags of required progression to initiate that event are improperly set, due to the open ended nature of story progression with each girl.
In my first playthrough, I was locked out of progressing Maria's events for almost 2 in game months before getting her back due to progression requirements to initiate that part of her story. Needless to say, it felt really awkward when I finally did get her back and read through the next few events.