Do you make sure that you dry the chips after boiling them? If they're too damp they take forever to crisp up. I spread them out on kitchen paper towel and dabbed them dry with another piece. Then I leave them for about 4/5 mins to dry out before putting them onto a baking tray sprayed with fry light.
But I do think that they take longer to cook than the mags and books say anyway. I cook them in the oven for at least 35 - 45 mins to get them perfect...also a very very hot oven.