A heavily alkaline sized paper might cause problems when teamed up with a dense negative, etc. I think the OP needs to determine all the various variables that may have changed between printing sessions. Time of day, time of year, any differences in negative density, paper changes, water changes (might be season differences in alkalinity of the water supply), etc etc etc.
Also the difference between an hour exposure and a two hour exposure is only one stop...not much, really. We were not told how long an albumin print took to expose properly. If it was 30 minutes, then we are only talking one to two stops difference.