Trouble installing 1950's with UN441 rails

I have finally obtained the correct shoulder nuts to hang Poweredge 1950 servers on my UN441 rails, but I am unable to get the servers to slide in more than an inch into the rack. Something is keeping them from going in. I thought it may be the shoulder nuts rubbing against the inside of the rail but that doesn't seem the case. I am trying to find the installation manual for the UN441 rails to see if I did something wrong, but I can't find it anywhere. Where is this document on the Dell website?