a few things that occurred to me...
1. individual differences - EW, lady x, and petra all explained very well how relationships with ISTJs haven't worked out for them, and EW especially in how it's simply not practical to keep pursuing that route for her. i know that personally, i experience a particular frustration with INFJs. i know a couple quite well and we just have a tricky time balancing things when we're together... somehow both of us feel like we're giving more than we're getting, and there's some kind of weird communication gap. i have a hard time seeing how a relationship with an INFJ could really work out longterm. i'm not ruling out the possibility, just saying that it's unlikely given past experience. if it's like this for other ENFPs with ISTJs, it's easy for me to see how it could seem like the ENFP isn't trying. it's hard when you always run into the same frustrating barriers, when those barriers are integral to a deep need you possess. demoralizing. and perhaps a large percentage of ENFPs experience that individual ISTJ frustration.
2. less consideration of the longterm before jumping into relationships - i think most ENFPs would really like to believe they can make it work with anyone, so they may be more willing than some other types (eg more practical ISTJs) to venture into a relationship despite warning signs and/or past experiences that suggest that things might not quite work out. this is true of Ps in general as well.
3. one-sided complaining versus figuring out what went wrong - as for ENFP complaining, a lot of what certain others think of as complaining / avoiding blame is us trying to figure out what happened. i used to get into this problem with my ESTJ boss. i would try to work out what happened to make me forget to turn in a paper or something, and she'd tell me to stop trying to rationalize it. but that was never my goal - i accept fault readily. i just wanted both of us to understand why it happened.