It is very common and not a fault for the centre (left and right) bottom of a radiator to be cool.. This is because the hot water flows into one end of the radiator and the hot water rises over the first 1/2 of the radiator, cools and comes down in the second 1/2 of the radiator. This leaves a middle section that has only a small amount of water flowing through the verticals and so will be cooler.