So far, I've never come across a puzzle that doesn't give enough info to finish the puzzle, and I've been doing this for a while. Sometimes the answers are in the grid, not in the clues. Sometimes making sure all your X's are filled in and looking for 4 X's in a row can help find the answer. Make sure you cross-reference all your completed answers, too, if you're really stuck.
When you're looking at the clues, there are some things to check for. We get really used to seeing that Adam is after Samantha, but sometimes it will say Adam is THE DAY after Samantha. If you're going quickly through the clues, it's also easy to mistake IS NOT for IS, and the other way around.