Not sure but you can try a different fertilizer. I alter mine.

What have your temperatures been? It is called "blossom drop" and there is a FAQ here all about it. Tomato blooms will only set fruit within a specific temperature range of day and night temps. Additional contributing factors are excess nitrogen fertilizer (from your description of your plants this may be part of the problem too) and inconsistent soil moisture levels.

You'll also find lots of additional info on it here by searching the forum using "blossom drop".

Hope this helps.

Dave

Here is a link that might be useful: Blossom drop FAQ

Agree with Dave. I dont have any experience growing Parks Whopper but I heard through some friends that it can be a stingy producer. Seems I heard that somewhere, or maybe not. Anyways, good luck!

for some reason I am distrustful of Parks. I had their catalogue once and it was so loud and splashy and in your face that it seemed like a cliche of a used car salesman.
Anything that shouts MORE BIGGER FASTER THAN EVER BEFORE just makes me shut down.
But maybe that is just me. :o)

Too much nitrogen fertilizer.