This impression may come from the fact that some INFPs tend to refuse indirectly.
Very often they won't say "no". They will be silent - because for them it may feel less disturbing for the relationship. Some people may understand it as "yes".
Hence the confusion.
Very often it's not being flaky, it's a way of saying no. The signals are there from the start but they are more subtle. That's why INFPs get compared to cats.
And why generally people refuse to help? Myriads of reasons.
- They don't have enough resources.
- They don't know how to.
- From their perspective you don't need help.
- They feel taken for granted.
- They feel used etc, etc.