I'll check my schedule.
In other words: "I don't know yet if I'll come, but I don't want to say NO right now".
Possible causes for that answer are:
- I'm not sure I'll have time.
- I'm not sure I'll enjoy this.
- I'm not sure I can put this on top of my priorities.
- ...and so on...
The other person may think that it's none of your business to know the real cause, so they may want to "throw a curved ball disguised as a fastball (1)". In all cases, it often involves these two needs:
- I need more time to think about it.
- I don't want to hurt you right away if I say "no".
I noticed this pattern over the years, either with family, friends, or customers. Of course, you won't handle different persons the same way. In your case (friends), I'd just add:
In case you're interested and find time, please let me know by [ day / hour ].
By doing that, I give them a chance not to answer without being rude. No answer, no party, no hard feeling. I never had the impression of putting pressure on the person, and never noticed them feeling uncomfortable. Never negative feedback either, so I keep it that way...
Additional Note: some people are too shy, so being 1-on-1 while saying "no" to a direct request isn't an option. OP confesses being "blunt" sometimes, and it may be the reason why people delay their answer. My answer is heading that way because I've experienced/witnessed situations like this one, but it's also important to say that some people just actually need to check their schedule :)
1. It's a metaphor to baseball. I don't want to try to hit the ball (answer the question). It could be an straight ball (easy question/answer) or a curveball (very difficult to hit, so, a tricky question/answer).