Single bolt seat clamp slipping
I have never had a problem getting single bolt tight enough to hold. 

Most likely is seat rail diameter - 7mm is common road, 9mm common MTB but there are others. (refer Wikipedia). 

Is there something assembled wrong, a piece upside down?  

Although seat post bolts need to be tight, what ever you do, don't be 'that' "gorilla with a spanner" - it is never the correct approach when assembling bicycles.