I think that u shuld make silkworms more common i grew 17 trees and got not even one i allmost starved to death because i didn't get enough food!!
Sometimes that's the luck of the draw, you should have at least gotten a few apples and 5-6 pieces of dirt out of that though. Which pack are you talking about?
It really is just luck based and I don't think MyM would change the drop rate since that is how the pack had it set to. Btw you should try using a crook ._. It took me around 30 trees using a crook on sf2 to finally get my first silkworm, after that I grew 20 trees next to each other and infested a single leaf to spread the infection to every tree and get atleast 5 out of it then grow more and MORE AND MORE so I could never run out of worm food