By default, Roku will only play HLS streams with a bitrate higher than 250kbps. To play lower bitrate streams, you should set the minBandwidth attribute of your content-meta-data object to a lower number. See the Component Reference, p19:
roVideoPlayer or roVideoScreen: Will only select variant streams with a bandwidth higher than this minimum bandwidth. Only has an effect on HLS streams. Default is 250 so that audio only variant streams are not considered by default. By default Wowza servers set streams to 64 kbs, so you might want to set this parameter to something smaller than 64 when first testing Wowza streams. You will eventually want to specify the Wowza bitrates with a smil file (Please see the encoding guide). Units are Kbps.