Leaks!

DS has been in his tots bots fitteds for months now, they never leaked. But in the last couple of days if it's on for more than a couple of hours he leaks, and badly - needs a complete change of clothes The nap is not always soaking wet either, just the front bit. I

I wash them in nappy san and tea tree oil, and use flushable liners so any bum cream etc doesn't come into contact with the nappy... any ideas why they've suddenly started leaking?

TIA