A frozen margarita is not a normal margarita with ice blended into it. The ice waters it down as it melts, so the mix has to start stronger and sweeter than you think, or you end up with a cold, thin slush that tastes of nothing.

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The one thing that matters
Use more syrup than you would in a shaken margarita. Cold flattens sweetness, so a frozen drink mixed to taste at room temperature will taste sour once it is frozen.
Blend two ounces of tequila, one of orange liqueur, one of fresh lime and half an ounce of agave syrup with about a cup and a half of ice for thirty seconds. Serve immediately in a salted, chilled glass.
Not enough sugar and too much ice. Cold suppresses sweetness, so a frozen drink needs more syrup than a shaken one. Measure the ice rather than guessing, and blend it fully so there are no chunks left to melt.
Blend and freeze in a shallow container, then stir with a fork before serving. It will not be as smooth as fresh, but for a party it is far better than blending twenty times.