Brian is correct on this. Some content creators have gone hogwild with the amount of commodities and combinations of commodities an industry can handle. In TS12 it didn't matter because there was no time-limit-checking so an industry or wagon could have a very large number of possible commodities. This sounds great, but this came at a cost as that causes awful stutters and other performance problems. T:ANE and up, however, puts a time limit on the loading, and that causes a timeout to occur.
You can verify this by turning on the script error reporting in the DEV tab on the Launcher or whatever it's called. (I can't remember the exact words and I'm not at my Trainzing computer ATM to find out).
There is, however, another thing to check in addition to the number of commodities being loaded. Check for faulty commodities and replace, repair, or delete them. If any one of these is faulty, it can prevent them from appearing in the waybill panel. It's been like this since at least TS2009.
