Usually under Sequence>Render I only have Needs Render (Video) and For Playback (Audio) checked when I am editing along and I use the shortcut Apple R to render what is selected as RED (Unrendered) in my timeline.
Under Sequence>Render All I usually have them all checked and only use the shortcut Option R when I take a break, go to lunch or have more time to let everything render. I then click Shift A to select all clips in the timeline then Option R to render everything in it. Most of our clips have Color Correction, Moves, Resizes and if I hit Option R each time I will be spending more time rendering and less time working.
Not being able to render may also occur because of a number of different things.
Here are some that come to mind.
If you have in and out marks on your timeline and your clip is outside the markers and not selected, it will not render.
If the colored bar in Sequences>Render is not checked, that color line in your sequence will not render.
I just highlight all clips I want to render and hit Apple R. As long as the color I want to render is checked in Sequence>Render I should be good to go.
Thats all I can think of for now,
Good luck,
Victor