I just make similar issue to work today....
I added a new pen to a working log and the pen display only when trend is open.... if you go out and open again the new pen just start and all others load the history....
It took me a week of digging on forums and resume that "trend does not read the string log file"....
All my working tags are REAL type in PLC programs and the new added tag was DINT type.... the DINT was logged into the string log.....
So I change the new tag to be REAL as all others in the trend and all just work like charm.....
So I resume that all tags on log should be the same data type.....
Regards