keep your supports on but give them a reason to bulk up the stem.
one way is to bring light closer, which you said you've done.
once light is not an issue another way is to stress the stem. take the support off sometimes, let the thing deal with it's own weight, even if it slumps over. leave it that way for half an hour or so then prop it back up and let it heal. once it gets where it can stand up on it's own twist the stem like you're twisting up a j...between your thumb and finger, once a day. twist and hold then let go and reverse, i did this to one of mine just to see what would happen and the stem is almost twice the thickness of the others and much, much harder.
another thing, which i've never done but it makes sense, is give it some wind when it gets strong enough to stand up.
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