There is no reason for that to happen.

The RDL may be bad, or a cable is bad.

If you don't have the issue with a direct connection to each projector with each cable, then it points to the RDL.

Likewise, it could be a cheap VGA cable - to long, not large enough.

I would pick up a high quality VGA cable and maybe a better D/A...

http://cgi.ebay.com/New-Extron-P-2-DA2-Plus-VGA-Distribution-Amp_W0QQitemZ230368578272QQcmdZViewItemQQptZLH_DefaultDomain_0?hash=item35a3098ae0

For whatever reason, it sounds like a cable issue then and you might have more luck if you pick up a couple of these...

http://www.monoprice.com/products/product.asp?c_id=102&cp_id=10201&cs_id=1020101&p_id=3573&seq=1&format=2

You really shouldn't be having issues with the cabling at 75', but you may want to try to double check by putting the laptop closer with a short VGA cable to ensure that the projector itself isn't having issues.