Pump robots seem to get stuck easily in the ground when pumping oil from world spawn locations. I setup a simple test with a natural oil spawn in the desert, where the robots worked fine for about 5-10 buckets worth of oil before getting stuck at the bottom of the oil spawn. They appeared to be stuck "in" the ground before collecting oil, but it was difficult to be certain. I also have seen this happen when I simulated a natural oil well (filled a 1x1 tube with oil buckets) after a few buckets were pumped/removed. The robot went down into the 1x1 and then about halfway down darted directly into the sidewall and became stuck. Both instances were in creative mode.

running Forge 10.13.3.1388 and Buildcraft 7.0.13 / 7.0.15.