Can anyone please tell me?????

Can anyone please tell me why when Im digging my spuds up the fork pierces the lovely big slug free spud but always misses the small ratty slug infested spud.

Murphy's Law. 

Tradition

Because the small, ratty slug-infested potato manages to slip between the tines of the fork, but the nice big fat round one can't.

I know from sad experience...

I used to have my Grandad's potato fork which had broad, flat tines, but that probably dated from before the WWI and eventually died of old age.  Now I start at the end of the row and put the fork in low under the root and seem to miss the tubers.  I also plant on a ridge in the first place.

Malcolm