You can only burn using the speeds listed in Supported Write Speeds field of the disc information: those are the speeds your burner and its firmware support for that media, and if you choose an unsupported speed, the burner will just ignore it and use the closest available speed. Also notice that the slowest speed doesn't necessarily result in a better quality burn, which is all that matters for reader devices.
Post the log of your latest non-working burn (look in ImgBurn's Help menu) and tell us why/how it doesn't work for you.