I didn’t have any luck with the adjustable-bar style feeder either. I had an orphan filly that I grafted onto another foaling mare; she still needed supplemental pellets and milk replacer, but the mare was a houdini about getting into any foal feeder, drinking the milk replacer and snarfing the pellets.
I had to create a “feeding area” that was short enough that only the foal could enter. In the stall, I had a fence board hinged to the wall that pulled out to block off a corner where the foal’s milk bucket hung (big foaling stall, plenty of room for mare without that corner). The board was high enough that the foal could go under easily, but the mare could not.
Outside in the field, I used three round pen panels to make a “box stall” that had a single 2x6" secured across the front, about 4’6" high. Again, the foal could easily go inside to eat her pellets, milk replacer, or alfalfa, but the mares were kept out. (other foals eventually followed her in, and sometimes snacked on the hay, but typically left the milk alone)