Why it happens
Roast potatoes go crisp when their surface dries out and fries in hot fat. If the tin is overcrowded, the potatoes release steam that can't escape, so they steam instead of roast. Fat that isn't hot enough soaks into the potatoes rather than crisping them. Potatoes that are still wet from boiling, or that weren't parboiled and roughed up, have less surface to crisp. Waxy potatoes also crisp less well than floury ones.