[milk] (the|a) (bull|ram)

to engage in a fruitless or pointless activity

The phrase "milk the bull" may have originated from the idea that bulls are male and therefore cannot produce milk, making the act of milking a bull pointless and impossible. This phrase is used to describe a situation where someone is attempting to do something that is impossible or to try to extract something from a situation or person that is not possible.

If we keep focusing on this issue, we'll just be milking the bull - it's time to move on to something else.
The politician's attempts to win over the opposing party were like trying to milk a bull - they were never going to work.
The project team is milking the ram by focusing on minor details instead of addressing the major issues.
The sales team was milking the ram by continuing to promote a product that was clearly not selling.
Trying to convince him to change his mind is like trying to milk a bull - it's just not going to happen.