Answer: Hollies will tolerate clay soil, so I do not think that itself is the problem. When you plant in clay, it is important to dig a wide hole (several times wider than the root ball at least) and not overly deep, just deep enough to set the rootball at the same depth as it grew in the container or a little higher. The soil should be loosened and the sides of hole should be left rough rather than smoothed off. The reason for loosening the soil is to allow the roots to penetrate outward more easily. You would also untwine or cut any encircling roots at planting time and direct them outward so they will extend into the surrounding soil. If these steps are not taken, the roots may stay in a circle in the original planting soil and eventually this stunts the plant. I would suggest digging down gently and see if the roots are spreading or still in the original circle shape. If so, I would lift them and replant taking the steps I described.
I am not sure what amendments you have been adding, but the most important things to do would be to water correctly and also maintain a layer of organic mulch several inches thick year round. It should be in a flat layer spread over the root area and should not touch the stem or trunk. This also helps feed the soil on an ongoing basis as it breaks down over time.
Other amendments such as fertilizer would ideally be added on the basis of soil test results indicating their need. Otherwise, you could use a general purpose granular fertilizer such as 10-10-10 each spring per the label instructions, or a slow release granular fertilizer such as Hollytone, again per the label instructions. It is actually better to underfeed than overfertilize, so do not try to push them by excessive fertilization. In your area I would expect the soil to be acidic naturally, that is fine for hollies.
We have had some wetter summers, and at this point they should be established enough to need watering only in dry spells in the heat of summer. On the other hand, a slope facing south can be hot and dry, so take care to check on the soil moisture levels, especially in the summer.
Also, if you need to replant them, make sure the soil is kept evenly moist (not sopping wet) this summer and fall while they become re-established. To know if you need to water, reach under the mulch and down into the soil with your finger. If it is still damp you do not need to water yet. When you water, water slowly and let it soak down deep. To see how effective your watering is, wait a few hours and then dig down to see how far the water soaked in. This can be surprising, especially with clay soil! It is better to water deeply less often than to sprinkle lightly every day.
Lastly, I should mention that hollies are not such fast growers normally. Less than a foot per year would be normal, these are compact growers and just do not shoot up fast.
I hope this helps.
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