Possible factors to explore...
1. Is there an orange line over the Timeline content that you want to Timeline render. The orange line is the program telling you that you do not have
the best possible preview in the Edit area monitor and, if you want that, you need to Timeline render to get that state. If the line is green or no colored line above the Timeline content, then no Timeline rendering is possible or available - pressing Render button just gets you playback.
2. Do the gray tabs of the Work Area Bar span the entire duration of the clip(s) that you want to render?
The above screenshot is from version 12/12.1, but it applies to 11 as well regarding this matter.
Please review and consider and supply more details.