This may be a 'working as intended' thing, but just in case; When multiple cows with different fluids are in range, if the rancher 'milks' a second cow before the fluid from the first has drained from the rancher, the cow's cooldown is triggered, but no fluid is gathered.

Yes, this is an issue I found in testing when I was first developing the mod. At the time, I was unable to figure out good way to solve this, so I let it be. However, I looked at it again and I think I found a solution. If testing goes well, I will put out a new build tomorrow.