One of the challenges of facilitating retrospectives in a training environment is that it inadvertently becomes the trainers' prerogative to act on action items. Its useful to remember that retrospectives are for the team and the trainer is just another member in the team. It therefore is important that all learners contribute to improving the learning experience.
Often it so happens that there's just a handful of people keep volunteering to drive action items. One of the tools I find useful to drive out action items is the "Who", "What", "When". I just draw up a flipchart with those headings and try to break up each action item into those fields. As a result, if someone has way too many action items against his/ her name, it shows up. Unrealistic deadlines are immediately visible too. I find this as a useful way to split responsibility and take action items to completion.