Yes, alas, personal issues do come up, so she may not be able to respond at the time. Some ladies stop communication if they have decided they aren't going to see a fellow, and that is their choice. I agree that it would be polite to simply say that a session won't be possible, but companions are under no obligation to give reasons. Probably is best to just move on to another lass.

My own policy is to reply and respectfully decline based on a specific point, for instance, the fine fellow is too young and I only see gents 50+. However, I have ignored some requests that were completely outrageous or offensive to begin with, and if someone persists in trying to persuade me, I will say no once more and then ignore/delete all further attempts to contact me.