iMac using vga muxlab blaun

We use a muxlab blaun to run 2 projectors using cat5 cable. The blaun works perfectly if we hook it to a PC, but when we hook it to the iMac using a mini dvi to vga adapter we do not get a display. We can hook a single projector that sits right beside the iMac to the same mini dvi to vga adapter and it works great. Any ideas? will the blaun work with iMacs?
Thanks

vga balun

check the model of vga balun.

If it is the 500010/11/14 then make sure shielded Cat5e cable is being used to ensure the required common signal ground. otherwise try the VGA balun II (500040/41/42)