DNA "refusing" to be run in agarose gel

[dilip]

I also had the same problem some time back and later found out as some 
others have posted that this is due to ethanol/isoproponol in the 
sample. Dry the pellet in a vacuum centrifuge (This will save your 
sample being sucked out) completely and then dissolve it in buffer.  
Also as another one pointed large amount of undigested high molecular 
weight DNA may also cause this problem due to less density and the 
problem is worse when trying to run High MW DNA just after 
precipitation. Hope this will help you.

Dilip