My guess is not enough light, and too much water (if you have watered every five days) and some rotted roots--that could explain yellowing at the base (then browning and dying) also symptoms appearing at the top.
I can't tell if the cute planters you have them in are free draining or not (because I don't see any hideous trays for drainage as we generally all have
). If they are not draining planters, I'll lay good odds on rot. If you've carried them to the sink to water every five days, I'll still lay odds on rot.
I suggest you check the roots of the worst one, not too much to loose at this point...you may still be able to save them (or possibly start over with something more suited for the window?)
