This could be a couple of things:
The usual suspect is that the viewport isn’t visible. I have my xpresso window tabbed up along with the viewport and picture viewer and the material nodes window and often I will pop in to the xpresso tab and wonder why it won’t update – because it’s taken over the entire screen with the viewport hidden in “behind”. The viewport MUST be visible for xpresso to update.
It could also be an ordering thing. The order stuff happens, both in the xpresso window (left to right) and the object manager (top to bottom) matters. If something has to be calculated first before doing something else, often it has to be left or below that second thing.
It could be that the data type is wrong. In the Spy or Result nodes make sure that the data type you are wanting to read is what that node is set to. And make sure that the data type is appropriate. This is often difficult as C4D seems to care sometimes (like Frames being Integers instead of Reals) and sometimes it doesn’t (Bools instead of 0’s or 1’s).
The final trick to try is to make sure you are really on frame zero and then stuff can calculate as it evolves. Just hitting rewind doesn’t always do it – often you have to hit rewind twice or, once at frame 0, advance to frame 1, then rewind again. C4D has a terrible habit of not clearing its own cache when a calculation needs to start again.