I've got the same problem, I've finally narrowed it down, it is either Nitrogen deficiency or potasium (K) deficiency. Both cause yellowing of the tips, however Potasium deficiencies cause them to brown and curl on the ends as well, yours aren't doing that, mine are
Anyways, I would suggest the next time you water those babies give them a quarter dose of some 20-20-20 fert. I'm using a 10-15-10, watered today but it looks like I overwatered. Hopefully it looks better by tonight.
Good luck bud