It may be that "It's been thriving where it's placed for years", but what have you been doing insofar as root maintenance during that period? Root congestion is a serious source of stress, and becomes limiting at the approximate point in time when the root/soil mass can be lifted from the pot intact. Once the congestion has been allowed to advance to that degree, the stress of root congestion will always be a factor. This is true if you pot up or even if the plant was planted out in the landscape. The only thing that will correct root congestion is a pair of hands in the root mass removing the offending heavy roots to make room for an additional volume of fine roots, the ones that do the lion's share of the plant's heavy lifting.

What most growers observe to be a 'growth spurt' after potting up is not a growth spurt. It's partial evidence of how severely the plant was being limited by root congestion, and at that, it's still a half measure. Full repots include bare-rooting, root pruning, and a change of soil. Where potting up ensures continuation of stresses related to root congestion already in play, repotting ensures complete elimination of stress from root congestion.

While there's not enough information to say exactly what's wrong, I can say it's a problem with root health. The primary suspects are root congestion/soil compaction, over/under-watering, a high level of dissolved solids (salts) in the soil (solution). Let me know if you want to chase this further.

I should mention that, even though I can't see the entire tree, I'm getting the distinct impression it badly needs pruning. Some are slow to prune because they are unfamiliar with how predictable the response is, others view any growth the plant puts on as inviolable, never to be touched, and proof of the grower's proficiency. Neither are true. Pruning, shoots AND roots, is a very important tool that will help us keep our plant eye-appealing, and help us guide it toward whatever vision we have for the plant.

Al

Al,

Thank you for such a clear and informative response. Indeed, I'm new to indoor trees and was glad to see this one growing well for some time and looking healthy. You're right, though; it likely needs a new pot and a thorough root inspection. I've repotted it twice and it's grown indoors to my ceiling. Before getting your message, I did prune it a bit and gave it a bit of fertilizer. I'll get a larger pot in the next few days. I can't thank you enough for the help!

The following images document the essence of a repotting session, albeit a more radical one than would be required of the average grower of indoor trees.

Ficus benjamina ^^^

Soil partially removed - note how coarse it is. ^^^

A very healthy root system, after bare-rooting. ^^^

Root pruning completed. ^^^ Note how large size of some roots removed.

Repotting completed ^^^

The following would be more like the 'average' repotting session:

Japanese maple

I actually air-layered this plant ^^^ off of the old roots - you can see them below the new roots I forced to grow - evenly spaced and radiating horizontally.

Here, the rootwork is completed, the old root system has been removed with a saw, and the heavy wire tourniquet used to create the new root system is still visible.

These threads should be helpful:

Overview of Good Growing Practices

Long Term Care of Ficus in Containers

Long Term Care of Trees in Containers

Best luck ~ Al