I have recently installed two new lavatory sinks, small bowls...11", with no overflow. I also installed a grid no overflow drain to go along with it, now the problem is that the sinks don't drain as they should, they drain slow. At first I thought something was clogged so I rodded everything out..... ALL drain lines, stacks... everything but still no luck, so I got a cheap drain from HD with a stopper and stuck it in, without the stopper, and the sink drained just fine but this isn't the drain that I want to use. I thought that the grid holes were to small, so I got another grid drain from Kohler and still no luck..... So I figured it has something to do with air equalizing or something like that. I guess my question is what can I do to make the sinks work properly, with still using a grid drain? Any help would be greatly appreciated!

Thanks!!!
Rob