I have a total of about 7 CPD (HP11) crunching. Since about a month ago I'm experiencing much, much lower blockrate. Currently it's about one block per 2-3 days. Wondering who finds all those blocks?

Also I noticed (since day 0) that my machines often find a few blocks in a row (over 1-3 days) and then find nothing for days/weeks. I even had 3 blocks within a few hours a few times. Can anybody confirm, that it is more likely to find a block when a block has been found in the past hours?  I can't believe this is pure luck.