It does not sound like a problem. 400-500W on a 600W dimmer would result in warm operation. You could consider going for a higher rated dimmer if you like, but you will find that it will also get warm. If I had to replace the dimmer, I would go for a more heavy duty one.
Experiment: Swap the "hot" dimmer with one of the other "cold" dimmers. Inspect the wiring while you're at it for obvious loose/bad connection. If the "hot" dimmer's now cold and the "cold" dimmer's now hot, it's possibly the load. If the "hot" dimmer's still hot, it's possibly a bad switch, replace it.
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