Easy Ruby, you nearly knocked him over! Any tips to get him to groom her back?

After her ruling the area and driving him away constantly, Ruby has now taken a liking to Harrison and grooms him (rather aggressively), but every time they end up in a grooming standoff (heads pressed against each other) she always relents until he runs away and she chases him. No matter how much she grooms him, he will not reciprocate. Any tips would be helpful!