DIAGNOSIS This is almost always a bad seal. The silicone gasket isn't seated right, is dirty, or has degraded and isn't making an airtight lock with the lid.
FIX First, power down and let it cool completely. Remove the lid and take out the main silicone sealing ring. Wash it thoroughly with warm soapy water, paying special attention to the groove where it sits in the lid. Dry it completely, then re-install it, making sure it's seated evenly all the way around in its channel. Before locking the lid, wipe the rim of the pot itself clean. Try again.
IF THAT DIDN'T WORK Check the pressure release valve on the lid. Make sure it's clean and moves freely. Sometimes a tiny bit of food debris can get stuck underneath it, preventing it from sealing. Gently clean the valve stem and the hole it sits in.
HEADS UP Those gaskets are wear items. If yours is over a year old and you use the cooker frequently, it's probably just worn out and lost its elasticity. You'll need to buy a replacement sealing ring specifically for the Universal Ultra 4.75 Qt model.