sugar carbing

I kegged my porter for the 1st time using 5L mini kegs. As instructed, I used 1tbsp sugar per keg. I waited 3 weeks. When first tapped, there was a lot of pressure in the keg, however the beer was ever so slightly under carbed. Tasted great, but no head. However, 1 day later, with suggested psi, it was ALL foam. I let it sit ater pouring and it was GREAT beer, but something went wrong. I have another brew that I used the same sugar and going to try less co2 pressure. I have one other mini keg carbing where I used the same sugar as bottling (hope the bung doesn't blow out). So we will see what happens. Any suggestions on how to sugar carb and co2 tap with better results?