Identify the problem with the following sentence. a few of the athletes who were not as worried about their weight would sometimes order calorie-laden desserts every now and then.

The word 'as' should not be in the sentence.
If it must be in the sentence, it would be:

A few of the athletes who were not as worried about their weight as (insert a noun here ) would sometimes order calorie-laden desserts every now and then.

Alternatively, instead if adding a noun, you could make it:

A few of the athletes who were not as worried about their weight as they should have beenwould sometimes order calorie-laden desserts every now and then.

Hope this helps!