In my experience, umpires almost never call obstruction for a defender impeding a runner. I see it almost every tournament as many 12U and especially 10U teams are taught to block bases as girls are approaching them with the intention of going to the next base. You'll see girls slow down all the time and the umps do nothing about it unless there is contact (not a requirement for obstruction, BTW). When it is called due to contact, then the umps almost never give the girl two additional bases, even if it was obvious she would have had them (and maybe more).
These teams also teach SS and 2B players to step into the baseline when a girl is obviously going to go from 2B to home or 1B to 3B just long enough to slow the runner down or to step nearly into the baseline until they see her slowing down.
Essentially, umpires have made teaching illegal obstruction into an advantage for teams. They can slow down runners without penalty and literally knock them down to keep them from scoring from 1B or 2B on balls hit in the gap or down the line. I have actually seen girls who started on 1B get knocked down by the second baseman on balls hit to the fence in the right-center gap only be awarded 2B after the obstruction while the girl who hit what would have easily been a triple is made to stay at 1B. That is powerful incentive to these teams to keep teaching illegal obstruction.
These teams also teach SS and 2B players to step into the baseline when a girl is obviously going to go from 2B to home or 1B to 3B just long enough to slow the runner down or to step nearly into the baseline until they see her slowing down.
Essentially, umpires have made teaching illegal obstruction into an advantage for teams. They can slow down runners without penalty and literally knock them down to keep them from scoring from 1B or 2B on balls hit in the gap or down the line. I have actually seen girls who started on 1B get knocked down by the second baseman on balls hit to the fence in the right-center gap only be awarded 2B after the obstruction while the girl who hit what would have easily been a triple is made to stay at 1B. That is powerful incentive to these teams to keep teaching illegal obstruction.