Sometimes, when you login to an account’s cpanel as reseller, there should be a drop down box to switch to the cpanel of various accounts under that reseller. If it is not present, you can check the following things.
In WHM -> Tweak Settings -> make sure the following option is not checked.
Disable login with root or reseller password into the users’ cPanel interface. Also disable switch account dropdown in themes with switch account feature.
Sometimes, the problem can be with only one or two domains. All accounts’ cpanel may be listing the drop down, but when you enter into one the drop down just vanishes.
In that case, it could be a problem with the account password. Try resetting the password of that account and see if that solves the problem.