Might be the soil mix. The volcanic soils, like a mix of pumice, lava/scoria, and akadama seem to form the best interior root growth that gets better over time, whereas organic mixes or fired clay mixes are a bit hit or miss. Also, when watering pines water REALLY thoroughly, then let dry out a bit. That way you'll be sure to moisten the interior of the root ball. Check to see if the interior areas stay bone dry. If so, your trees will appear to dry out terrifically fast. Soak your pots in that case from the bottom up to restart penetration from above into the interiors.