It sounds as if they are being held back by something once you plant them in the ground. Make sure that the soil is well dug and add extra compost for good structure. They may not be making a good root system if the soil is clay or hard lower down. Water every few days and feed well. Newly planted plants are still effectively in a 'pot' until their roots have had time to grow out into moist soil so they are reliant on the gardener for water and nutrients for a few weeks. Are they being planted in the right situation? Sun/shade etc? What sort do you grow?

Also, if they are blooming when you plant them, they may send all their energy into seed, or a few more flowers, rather into the roots that will support a larger plant, with even more flowers. If you can stand it, it is better to pick off all blooms and visible buds when planting. Another common mistake people make is to plant their flowers too deep--about 1/2 cm of the original root ball should be showing when you are done planting. "Plant it high, and it won't die. Plant it low, and it won't grow."

Thanks. I had not heard of the idea of planting high. I don't plant really deep though. I do pinch out when plants are young. I may not be digging well enough or maybe I'm letting them get pot bound.

Well, it only takes a half centimeter of soil against the stem to stunt or kill some types of annuals. As for pot bound, that would be visible when you plant them out: lots of white roots winding 'round and 'round the rootball. it can be treated by pruning both the roots and the top when you plant, but that will slow the plants temporarily.

you could also test the ph level of your soil, if it is too acid this could prevent nutrients from being taken up by the plants. i had the same problem a couple of years ago and solved it by liming