Some systems cannot access NLB address, others can

I have a set of Windows 2003 SP2 servers running NLB for use with an IIS FTP
site (the data is directed to a UNC path file server). These are two Dell
PowerEdge 1950s with two NICs each. As far as I can tell, the load balancing
is setup correctly, but for some reason, some computers can access the load
balanced IP address, some cannot, even within the same subnet.

All the systems can ping the nlb address, 10.1.1.218, but only some can open
up the ftp site. If I go to either system's address, 10.1.1.184 or 185, I can
log in with no issues.

I am not sure what is keeping some clients from accessing this and allowing
others. Has anyone else seen this?