If you are applying reverb via the Effect fader in Compose's Mixer (choose the Playback menu and select Show/hide Mixer panel, or press SHIFT+M), there is a known issue where WAV and MP3 files exported from Compose might begin with a short duration of dry audio before any reverb is heard.

Currently, the only known workaround is adding a blank measure at the beginning of your document prior to exporting.

To add a measure, select the first measure of your document; the Edit selection panel appears on the left side of the screen. Type "1" into the Before box of the Add/remove measures dialog and click Apply.

Alternatively, you might consider exporting your audio file as completely dry and then importing it into another digital audio workstation (DAW) to apply reverb there. To do this, you can either turn the Effect fader all the way down or select None for the effect type.