Jetpack Compose Preview not showing

I seem to be having trouble with Preview in compose, the layout panel doesn't appear when I annotate a compose method with @preview. I assume I'm missing a dependency, but I've copied and pasted the code from here https://developer.android.com/jetpack/compose/setup. Any suggestions? (tried the usual clear cache, reopen project etc) :)

buildFeatures {
        compose true
    }
    composeOptions {
        kotlinCompilerExtensionVersion '1.0.0-alpha10'
        kotlinCompilerVersion '1.4.21'
    }
}

dependencies {
    implementation 'androidx.compose.ui:ui:1.0.0-alpha10'
    // Tooling support (Previews, etc.)
    implementation 'androidx.compose.ui:ui-tooling:1.0.0-alpha10'
    // Foundation (Border, Background, Box, Image, Scroll, shapes, animations, etc.)
    implementation 'androidx.compose.foundation:foundation:1.0.0-alpha10'
    // Material Design
    implementation 'androidx.compose.material:material:1.0.0-alpha10'
    // Material design icons
    implementation 'androidx.compose.material:material-icons-core:1.0.0-alpha10'
    implementation 'androidx.compose.material:material-icons-extended:1.0.0-alpha10'
    // Integration with observables
    implementation 'androidx.compose.runtime:runtime-livedata:1.0.0-alpha10'
    implementation 'androidx.compose.runtime:runtime-rxjava2:1.0.0-alpha10'

    // UI Tests
    androidTestImplementation 'androidx.compose.ui:ui-test-junit4:1.0.0-alpha10'

    implementation 'com.google.android.material:material:1.2.1'
}

Here is my attempt at using preview (in AS it says Function "DefaultPreview" is never used)

import androidx.compose.ui.tooling.preview.Preview
.....
@Preview
@Composable
fun DefaultPreview() {
    Text(text = "Hello!")
}

from Jetpack Compose Preview not showing