Have a look at how the calves are 'kissing' each other after feeding. It's a typical sign that their suckling needs aren't being fulfilled.
A good solution is to use a teat: the calves drink slower, so their suckling need is more likely to be fullfilled. To compare drinking speeds:
From bucket: 10 l/minute
From teat: 2 l/minute
From mother: 1 l/minute
More info: Why calf feeding with a teat is better