I bought 4 brand new Toyo PR1's about 5 weeks ago, done about 1000miles in them, nowt special, just to and from work.

Now over the Xmas period i notice that one of them is soft (not flat), so i put air in them all, 30PSI, then the next day the same tyre is soft, it went down to 20PSI. Check the tyres for nails etc....nothing, its mint

what could be causing this, do you reckon it could not be fitted correctly ?

 

Might not be sested properly, slow puncuter that you've missed(able to repair these if not too close to sidewall) or dodgy valve could all cause the above.