Yes I have notice this fluctuation myself and I also seen a good youtube video about it proving that ebay does indeed sometimes show your items and sometimes doesnt. They did this by creating sales graphs for various accounts and then tracking when sales picked up and when they slowed down. There was never any consistency which should be expected. It always spikes for a few days and then down to nothing for a few days. And I also get the same location wise, a different area at a time. One day ill have loads of offers or sales from people in France, etc etc. 

Annoyingly ebay have decided that the day to show my items is on a Monday. I always do rubbish saturday sunday when it should be peak selling time in my market, and then I always have a brilliant Monday. Its not too bad because I like having a good start to the week but I still wish my items were just shown consistently. I refuse to pay promotion, never had to and never will.