13 mph is pretty low for 300 rpms. That makes you very rev dominant,
and is mostly the cause of what you're seeing, unless a house puts down more oil than average.
Most houses, especially during the summer, will try to get away with as little oil as possible, oiling as infrequently as possible. One local house will ONLY oil before a league. If there's no league for 3 days, they're not going to oil at all.
If I were you (and I was), I'd try very hard to either raise that ball speed or lower that rev rate. Otherwise you'll be using urethane or balls, like the Slingshot or the Silver Recon, for a long time.
"None are so blind as those who will not see."