Well, there is a thread like this under parkour techs, so you may want to look there.
Also, there could be plenty of reasons. Are they older / larger than you? It could be a matter of physical strength, or it could be something to do with the way you / they are doing it. It could be you need more confidence or determination.
What I would recommend, whatever the cause (unless it will damage you) is practice, practice, practice with someone watching, and more practice after that. Keep at it.