January a year ago I planted my peppers, I put one in a big pot & it produced big peppers all summer. When it got cold we brought it inside & have had it under the grow lights since. Until about two weeks ago it was doing great, I was getting peppers off of it every week, but now the flowers turn brown & fall off & the leaves are turning too. I've sprayed it with Neem oil which seemed to help some. Interested in some advice.
This is another example of changing the environment for plants too quickly. For plants grown indoors, it takes a good 2 or 3 weeks to 'harden' them off to be able to live in the out of doors. For plants grown out of doors it takes 2 or 3 weeks to 'harden' them off to live indoors. This is your only problem. The artificial lights are great but they are still different than what your plants were used to.
Neem oil is an insecticide and also a way to 'gloss' plants. This glossing slows transpiration in a drier environment. That would have helped with the drier environment indoors but your plants are quite weakened and you might find lots of spider mite or white fly if not now soon. Otherwise, your use of Neem shows (grins, shame on you for not reading the information sheet) you were just at your wit's end. Unless you actually see and identify a pest, do not use any pesticide!
Sorry, we pesticide applicators were taught 'read that label 5X each and every time you use that product'...not what works or what doesn't!! Great program but I am serious.
I've found pepper plants to be incredibly hardy to the point they can actually handle being covered in white fly and spider mite and still produce peppers. Your plants were just holding onto the peppers produced before you brought them inside.
I'd start new peppers right now under those lights (should be blue spectrum and to get them flowering go to red after a couple months). Harvest your peppers and allow them to dry or roast them and your new crop will be coming along quickly. Peppers are one of my absolute favorite crops!! Just a few and only in 4 to 6" pots produce so much. Get them started in TINY pots first and then transplant to 4" then 6". Indoors, use ONLY potting soil. Low nitrogen when you want them to go into reproduction phase with the red spectrum lights! (14-14-14 or 5-7-9, the N being equal or lower than the last 2 numbers). Keep moist but drained well and definitely not soggy. Also you HAVE to have a fan blowing 24/7 to reduce any fungal problems...hope this helps!!
Your plants could be going through a little bit of shock after being moved indoors. However, it sounds to me that your plants have been indoors for a while now, are probably acclimated to their new environment. Seeing as it is nearing the end of winter in my hemisphere, they probably have been inside for a few months. Moving plants from inside to outside is far harder for plants to cope with than moving outdoor plants to in (especially if they are given adequate artificial light). I never "harden" off plants to go inside, but if you don't harden off indoor plants to plant outside, they will suffer and possibly die. Therefore I'm hesitant to assume that is the explanation.
Have you been hand pollinating your pepper's flowers? Many flowers that don't get pollinated will fall off and won't produce fruit. Unless your house is teeming with insect life, you are going to have to hand pollinate those flowers to have reliable fruiting.