Obstructed runner contacts defender

[Thurman15]

Umpires out here this season have not been calling any OBS or INT calls if there is no contact. I've seen dozens of games and neither has been called if the players don't touch each other. I do mean literally not one, single time.
So as a result some coaches began telling their players mid-season that they had to contact the player in order to get the call. So they have been doing so since then, without malice, and have been getting the calls instead of no calls, and there's been no USC calls.

What I can't figure out is why it took contact for the umpires to call OBS when the runners' progress was clearly impeded on several occasions that I witnessed, but never called. Or fielders with first play on batted balls seemed to be interfered with by base runners, but INT was never called until things changed and the fielders started to make sure that there was contact with the base runner. It's almost as if the umpires were afraid to call OBS or INT without obvious proof (contact). Now with the contact between two players the calls have to be made.

The more experienced, aggressive teams, with heads-up players have worked this to their advantage by initiating contact or putting themselves in the way, while the smaller, more meek teams' players still avoid all contact both ways, and never get the calls made.