Correlate does not always catch all differences in a script.
Solution
Recently I noticed, in the LoadRunner forum, that when people ran a correlate on a LR script and found no differences, but the script still errored, they became stuck as to what to do next. Correlate does not always, and I have come across quite often, find all differences in a script. The best way to find the session ID's etc. is to save the original script, create a new script following the exact same process and then do a Vuser compare under the tools menu. Then you can create parameters for the session ID's etc.