I was given a 6L stainless Smartplus as a gift to replace the (very) old alminium Prestige model I'd inherited. I used the old one loads and had always found it trouble free. I was very disappointed with the new model which had many of the problems described in other poor reviews, it took a long time to come to pressure, a long time to cook food and used ridiculous amounts of water, often meaning the food inside became dry and sometimes burned.
On washing the lid after another disasterous attempt I managed to remove some 'material' from within the safety valve. It could have been food but I don't think it was. After this the valve moves much more freely within the rubber seal, it now falls open or closed as you turn the lid upside down and back. Previously (I think from new) there was more resistance to the valve moving and you had to use your finger to move it through the rubber seal. The pressure cooker now works brilliantly, if the contents are boiling before I put the lid on it comes up to pressure almost immediately, only a very low heat is required to keep it at pressure during cooking and virtually no water is used.
With it working as well as it is now I've not noticed any real difference between the 12 psi pressure and the old 15 psi of the aluminum model for the cooking I do. If you're having problems I'd suggest a look at the safety valve or maybe try a new one.