Why is cathether ablation unsuccessful?

My son had an ablation in January.  All was clear at the 1 month follow up, but his EKG was suspicious at the 6 month.  He just had an SVT episode which confirmed that he will need a second ablation procedure due to WPW.  Why was the first one unsuccessful in the end?  Is it common to have to have a second ablation?    What creates the need for a second ablation?  Did the first one not take, or did another pathway occur?  If it doesn't work this time, what are our options?