Answer: There are several possible reasons why this is happening. One is that you are planting too shallow or not getting good rooting due to poor planting technique or to the plants being rootbound. At planting, loosen or cut any encircling or badly matted roots and then plant at the same depth as it grew in the container. The hole should be dug about the same depth as the rootball and much wider to encourage roots to spread outward. When planting into heavy soil, the edges of the hole should be left rough. Snug the plant into the soil and firm it around the roots as you refill the hole. You don't need to stomp on it, but press firmly. Then water thoroughly to settle any air pockets. Then you need to keep the soil slightly moist (damp like a wrung out sponge) to help them root into the surrounding soil.
If you have used copious amounts of organic matter during your soil preparation and plant immediately after tilling or extensive digging, it is possible the soil still needed time to settle. Once this happens your plants may be a bit too high. I would dig down gently and see how the roots are growing, if they are still growing in the original potting mix they came in for example then you know there is a definite rooting problem. You can try lifting them and cutting the encircling roots and replanting. If you do not do this they will not thrive, so it is worth a try. If they are rooting out into the surrounding soil you can try topping it up a bit, do it gradually about a half an inch at a time. Using a mulch will also help. You do not want them to end up too deep - a little on the high side is better than a little on the low side.
I hope this helps.
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