I tracked down the problem to incorrect file permissions for the python script, inadvertently introduced during editing on Wednesday.
The original script ran as a cronjob at 11:36 for CP3 and 11:37 for CP4. Both were fast fills, here are the logs:
Starting CP3 fill. LLCV enabled. LLCV set to manual control. LLCV set to 50% open. Fill completed in 21 seconds. TC B did not register fill. LLCV set back to 20.0% open.
Starting CP4 fill. LLCV enabled. LLCV set to manual control. LLCV set to 70% open. Fill completed in 18 seconds. LLCV set back to 34.0% open.