In fairness to Bob, he will attempt to pit for tyres and/or fuel when required. The problem is: at the Ring, pit lane is near the end of a very high speed straight. Therefore, if the tyres have gone away, Bob may skid past the entrance of pit lane, bash the armco for a while, then give up & continue the next lap. Chances are, he will make the pit lane entrance next time around, (slow him right down if you are there to moniter this).
Luckily the AI have a similar problem, so things tend to balance out.