Why does my Rules Action about booking a time slot appear to not be executed?
I'm about sure that the answer to your question can be found here.
And to be 100%, I recommend you do some Rules debugging, as explained in the answer to "How can I display the value of a variable inside a condition?".

Update

Your (1st) comment confirms what I wrote in my answer even more. Have a look at the source code and let me know where I can find more details about any of those "Various events" that you try to use are actually coded. I trust they are somewhere (since your question says you use them), but with my limited knowledge of Rules (hm, hm) I cannot find them ... in the typical ".inc* file where the Rules module expect them to be located. But since "I" cannot find them, it may well be that that also explains why the Rules module cannot find them ...