Make sure the space around them is completely free. They take up five squares, like in a cross formation, and if any of the squares aren't empty (disregarding the monster you're fusion-ing into the main monster obv), it won't give you the option.
Which means a lot of cramped maps you can't even use Fusion, argh.
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"Duel audio? You mean like rap battles?" - termina_rzident
KrahenProphet and Kana are on opposite ends of the Awesome Spectrum.