I make it a point to not make concrete plans... because honestly, I'm likely to not stick to them. I'm really about doing what I feel in the moment, and if I make plans to do something a week later, I may not be in the particular mood to do it at that time when it comes up. I may rather be by myself, or if we planned to go shopping, I may rather go for a hike or read a book or simply be lazy when it comes up. Who knows?
Sometimes I avoid activities I think will make me feel uncomfortable.
Sometimes, because I do prefer to go with the moment, I may make plans to hang out with someone one day, but the day before might get so worn out from unexpected last minute social interaction, that when it comes to fulfilling my plans with a friend, I might simply be too worn out from all the social activity and need to recharge.
I think a lot of my extroverted friends have gotten used to this, and usually just tell me to give them a call on such and such day if I feel like doing such and such activity and that we'll work it out from there. This is usually the best option to work with, allowing me to have more control in the situation and them not being tied down to the plan either in the case that I decide to do something else. This way they're allowed to make other plans, too.
It's not that ISFP's, INFP's and INTP's are shy or flaky, we simply need more time to be by ourselves and recharge and want to only involve ourselves in situations we know we'll feel comfortable in. If I'm not fully recharged, then I won't be much fun and therefore won't feel very comfortable. :P And because we're perceivers and not judgers... schedules are kind of a fail.