Your ice is too cold. Little icy parts on the surface will give the bubbles points to form on. Bars always have ice on the verge of melting. Also a dry glass with have more opportunities for the gas to come out.
Try putting your ice in the glass, putting some water in, swishing it around so that the surface of the ice is now warmer as its cooled off that water and frozen some of it and the glass is wet then tip out the excess and see how you go for flatness.
This is part of the reason that bars have those glass washing things at the sink.
Also are you pouring a warm can into ice or is the can in the fridge already?