Your plants are growing through the winter, not dormant, so they still need fertilizer. Without it, they will not have the nutrients they need to grow and sustain fruit. Fertilize as you would through summer, at a reduced strength. You do not need to hand pollinate these. They will do all of that themselves. Citrus blooms contain both male and female sexual organs, so don't need any external help. If the tree is young, it might not support much fruit. It usually takes the third year of fruiting to get a good harvest. In the meantime, you must provide the plants with proper growing conditions. The light is fine, and you seem to have the watering right (just don't let it completely dry out!!). What about humidity? They like humidity in the air, so gravel trays would help. They also don't like highly peat-based soil mixes. A 50/50 mix of potting mix to inorganic matter (course sand/turface/perlite etc) would be fine for a spring repot. Check the roots are not too constricted. Falling leaves indicates Winter Leaf Drop (WLD) so do a search on the forum for this and how to avoid it.

Meg, it's true that a certain percentage of immature fruit drop from a tree, but I think it happens moreso changing environments. Just like many flowering plants drop flowers once they come home w/us. Also, a NE exposure may not be enough light for citrus,...a tree that devours sun. Is it possible to set outdoors in summer? If not, do you have a brighter window? South or west? Otherwise, your doing everything right. Good luck, Toni

To answer the original question it seems as tho the plants are just super stressed, and when they get stressed they drop their leaves and fruit to survive. They are getting much different conditions than they were at HD, where they outside or inside a greenhouse in HD? I am going to guess that they were inside or at least shaded, they had to be hardened off in before just putting them in full sun. This is done by SLOWLY introducing them to the sun for a short period of time gradually working up to full day sunlight. Stop fertilizing and watering until you know for sure they are dried out. Than completely soak and repeat the cycle. Citrus will survive slightly dehydrated better than they will flooded. The meter you are using measures the electric conductivity of the soil so if it is littered with fertilizers you will get false readings. Are you able to post pics of the trees and their pots? I would suggest a re-potting and pruning, but as you said you are new to gardening and this may scare you a bit. Do you have any friends with a green thumb that could help you with this? Don't expect any flowers or fruits till next year as said since the plants are so stressed, but you never know with some TLC and luck you may get some winter blooms. Again pics would really help, and good luck!

Most flower set fruit but drop. Hopefully your tree will hold onto 5 or so lemons. Steve