My girlfriend was in the bath on her knees over a bucket of gunge full of apples. Hooray. Unfortunately, she couldn't successfully bob any of them. I gave her no time limit and she still gave up at zero apples.

Obviously, for failing, she got gunged. But I want the challenge to be a bit more competitive instead of impossible.

Was the gunge too thick? Is it easier with thinner gunge? Was she really bad at it? Something wrong with the apples? (The apples did float to the top) Any advice?

Try using water instead of gunge (in the same bucket). Can she do it then?

Apple bobbing is difficult enough with just water - lubricating the apples certainly won't make it easier. Perhaps try something sticky rather than slippery - I don't think the thickness is the issue as the lubricated surface of the apple will stop one getting gnashers in.

Maybe a syrup? Treacle? Perhaps icing if you can ensure it isn't gritty (you'll want to avoid grit in the eyes).