Beat is right! For an ISTJ, exposing yourself seems to be like breaking the ISTJ code of silence! It is very awkward and to do it in person (in public) would be VERY rare.
But if YOU are trying to tell an ISTJ something, are you using VERY obvious (submissive?) body language? If not, then you may come off as not truly meaning what you are saying... If you say you are interested in an ISTJ, that ISTJ needs to see some concrete evidence-look into their eyes when you are talking? If they hesitate to answer your question, give them time- don't back them into a corner and force them to answer because it will probably back-fire on you. (I explained what the response might be like in a previous post)
I'm not sure what to tell you- except that an ISTJ needs evidence of blantant and obvious body language or other means of getting the message across- not just you saying it to them. Oh, and we like boundary's so im not saying go try to violate your ISTJ because that will just push him away and make things A LOT worst. But the closer "friend" you get to the ISTJ, the more and more boundaries/layers come down. Ever seen SHREK? We are like onions- we have layers! And you are trying to get to the core.
Perhaps by writing an e-mail or texting? I know for a fact it would be a cold day in hell before I ever got involved in a conversation like this in person- I would most likely walk away!
Gosh my posts seem long- we need a live chatroom on this website!