(This is from another reply, but I'm actually quoting myself, and I can't really improve on it...)
Often this happens when you don't realise it, but you've left a tiny fragment of a clip at the end of the timeline. The best way to check for this is to click on the 'zoom out full all axes' button at the top right corner of the Editor display. If the large gap is still there, there's somthing on one of the tracks you need to remove. An alternative is to select just the amount of the session you want to use, and do a mixdown selecting Time Selection as the option.