Proper application of OBS?

[U of M Sam]

12U LL softball game. Bases loaded, 1 out. Batter hits a "shot" to left/center gap. Runner from 3rd and 2nd score.
Runner from 1st is on way to 3rd when F5 stands on F6 side of 3rd base blocking base completly, without posession of ball, prior to runner from 2nd reaching 3rd (F7 is just picking up hit ball). I am PU and signal OBS. Runner coming from 2nd slows down, touches 3rd, starts to proceed to home then stops and returns to 3rd. ball from F7 is received by F6 on the infield. Play stops when pitcher has ball and coach calls "time" to question why his runner was not awarded home because she "would have made it anyway". I had runner from 2nd protected to 3rd not to home. Correct call?