Answer: Based on your description I am not certain what the problem is; it could be any one or a combination of those you listed.
These trees do best in soil that is acidic, organic and humusy, and evenly moist like a wrung out sponge. It should not be sopping wet or saturated, but it should never dry out.
Using several inches of organic mulch over the root area year round can help keep the soil moisture levels more even. The mulch also feeds the soil slowly as it breaks down over time. You do not want to mulch too deeply as this will prevent air from reaching the roots or encourage the tree to root upward toward the air.
They do best in morning sun or very bright dappled light all day. All day sun can be okay if the soil is adequately moist. Afternoon-only sun is too strong and harsh and can contribute to stress -- especially in the heat of summer.
Overfertilizing or using fresh "hot" manure could cause "fertilizer burn" which would damage roots and show as foliage tips turning brown. Accidental herbicide exposure or chemical spray could also cause this.
It could also indicate that they are not rooting out into the native soil, if either the soil is not suitable or if they were potbound at planting time and the encircling roots were not cut and directed outward.
But, new trees need several years to become rooted and established. So to some extent what you are seeing could be a result of transplant stress.
It is not easy to diagnose this type of situation long distance. I would suggest you consult wiht your local professionally trained and certified nurseryman, to make sure they are progressing within the realm of normal and make any adjustments if needed. If they seem to be thriving apart from the tips, I might be inclined ot leave them be and see how they do next summer. An exception would be if they are planted in afternoon only sun in which case I might consider transplanting them this fall.
I hope this helps you troubleshoot. Good luck with your trees.
Q&A Library Searching Tips