Low levels of potassium and phosphorus are one of the contributing factors to this, as is too much sun, believe it or not, along with some genetic factors. If the plants with the problem are the same varietyof tomato as the ones without, then I'd assume that they're either getting too much water, and the feed is therefore passing through the soil too quickly, disabling take up, or the feed ratio to water is too low for these. You're right, I think its the autopots.

thanks for that bamboo,have just turned water off in the autopots will now start watering/feeding as normally do from the top.next year will stick with normal pots feed/watering,stick them in containers,and just fill with water to last for the week and drain off on return as ive done other years and get some protection from the sun.yes they are all the same variety,gardeners delight love them nice and sweet.once again thanks.