Do you mean 'come' like when she darts out into the yard by acident or 'come as in when you are working in obedience? The first one I'm still working on heheh..
The second one, don't use stay.. use wait. You want her to stay when you tell her to. You want her to relax and just stay there. When you tell her to wait, you want her all ready to spring.. she is just waiting for the slightest start of 'come'..
Do the same thing, put her in a stay/wait.. step out a couple steps and then tell her 'come'. If she moves before you say come, place her back in the sit/wait position right back where she was tobegin with. Try to avoid repeating the command. Again, keep the training short, keep it light with lots of praises and fun.