To be totally honest I do not know why your dog should have been less responsive to the whistle in sugar beet than other cover!
All I can put it down to is a form of disobedience possibly caused by the level of game scent beneath the canopy of leaves and the presence of other dogs also getting warmed up on the same scent.
The other possibility is that when your dog was in training and working in sugar beet you allowed it to get away with ignoring commands, or you were inconsistent in preventing misdemeanours.
I suggest you go back to basic whistle training to put this problem right.
At the same time I would also make the point to other people bringing on a young spaniel that they should ensure its early hunt training takes place on barren ground with no scent distractions.
This way they will avoid the problem of ending up with a spaniel that?s reluctant to hunt unless the ground carries fresh scents.