Some LED bulbs are sensitive to which wire is Positive and which is Negative. It could be as simple as plugging in the 921 replacement LED and, if it does not light up with the switch, pull it, turn it 180 degrees and plug it back in. Let us know if that fixes the problem.
I use LED panels (36 or 48 bright LEDs in a small square) in my ceiling lights. Those put out tons of light (almost too much). I'm not sure how many LEDs are on your 921 replacement but I'd guess something like a dozen? It may just not be as bright.
I got my LEDs off eBay. I don't recall the seller (but could look it up if you PM me) - I know he was in Hong Kong. I paid about $3.50/panel, each were quite bright, each was "warm white" color (which made the wife happy), and each has been working for 3 years now. I'm real happy with them.