some strains, mostly indica doms i find, the buds at 4-5 weeks or so look pretty sparse, they have clumps of buds forming but do not meet up until week 6 onward
from week 6-9 they pack on all the weight this is when the buds meet up and colas are formed i call these late bloomers
skunk strains that are sativa doms can often produce colas right at the start early bloomers at 4 weeks the plants have colas all over them
or it maybe just a lower yielding pheno that does not produce colas at all, but clumps of buds covered in heavy resin some of these plants can make very potent personal keepers
hope it all works out
peace