That's a bummer, especially when they seemed supportive to begin with. Have they seen you in female mode yet or are you still presenting to them as male-ish. If so it could be they are having trouble conceptualising you as the opposite gender. I know I copped resistance from some friends the first time I tried this...until I started going out en femme. Until they see proof then it may seem to be a bit of a "crazy idea" that they might not want to encourage. Just a thought.
It could be any other number of things of course, but it sounds like maybe they're uncomfortable talking about it because they don't understand what being trans* really means for you, and that they're maybe even a bit afraid of what transition holds for you and them.
Personally I'd suggest you just grab the bull by the horns, don't try to second guess what they're thinking by their behaviour, just say "I feel like you're uncomfortable about my gender issues and my desire and intention to transition, is that the case? Is there anything we can talk about that will help ease your concerns?" They sound like reasonable people and hopefully they'll be more open - don't expect to hear things that you'll like or that will make much sense - but it might make things a bit clearer in the end. Just stay as calm as you can manage.
Good luck whatever you decide to do!