I don't keep many plants as a personal rule, generally only five or six at a time. I'll have one of each size plot in regular and enchanted soil. Lately I've noticed the 48 hour dragonfly deterrent for levels 1, 2, and 3 has been failing for at least one plant in that time frame. Twice I've had levels 1 or 2 pests while dragonflies are present. Is there something I don't know or is this a true buggy bug?
I am going to make an assumption since you have many different pots and sizes.
That is you do not plant all your plants at the same time. When using other pest spells such a bug zapper if I add a plant after casting the spell the new plant does not get bug zapper applied to it. Only the plants that are growing when the spell is cast get taken care of.
If this is true, and i think it is, you added a plant after casting dragonfiles, this plant will not be protected, even though you see them flying around.
Also, I have tried many plants and so far ALL plants work in enechanted soil so you can get rid of half your pots. I plant everything now in enchanted soil.