I have this same problem. Extremely frustrating. I have an old maroon clown (had it for 10 yrs now) hosting with a GBTA (that I just got a month ago) that will not let the nem get a single bite of food. I always try to feed the other side of the tank with mysis first to distract the clown and fill it up before squirting some on the nem, but without fail, the senile/severely retarded old clown will brush every bit of food off the nem and then eat it. As a result the GBTA isn't doing well and is getting smaller and smaller. And not to mention, I have to do a lot more water changes due to all the excessive feeding.
Again, extremely frustrating. The GBTA is in a place on the rock that makes completely covering it with a basket impossible. At this rate, it will likely starve to death.