From the photo, there may be a lack of mycorrhiza. In Pinus, lack of mycorrhiza is first seen in the nursery and, later in the field, the plants just do not thrive. If they obtain the mycorrhiza in the field, they will suddenly grow, although others may still be stunted. A solution is to look for puffballs under well grown trees, make a slurry of the spores (rub them into dishwashing detergent first to make them disperse in water) then water the slurry over the seedbeds. Puffballs are easier than the mushroom-type mycorrhizal fungal species as it is possible to obtain large amounts of spores very easily. Also, look at the roots of healthy plants and see if there are any coralloid roots as an indicator of ectomycorrhiza.