It could be any of the following reasons, which is described on unity doc.
please check with this.
Android Notes
* Support for resolutions above 640 x 360 is not available on all devices. Runtime checks are done to verify this and failures will cause the movie to not be played.
* For Jelly Bean/MR1, movies above 1280 x 720 or with more than 2 audio tracks will not be played due to bugs in the OS libraries.
* For Lollipop and above, any resolution or number of audio channels may be attempted, but will be constrained by device capabilities.
* The Vulkan graphics API is not yet supported.
* Format compatibility issues are reported in the adb logcat output and are always prefixed with AndroidVideoMedia.
* Also pay attention to device-specific error messages located near Unity's error messages: they are not available to the engine, but often explain what the compatibility issue is.
* Playback from asset bundles is only supported for uncompressed bundles, read directly from disk.