When you review delegated actions at the next meeting, peer pressure often helps to encourage people to complete their actions when they see everyone else getting theirs knocked of the outstanding list. (This is one reason why having structured meeting notes and reviewing them at each meeting is such a useful tool).
If you find someone persistently doesn't complete their delegated actions, you should first speak to them privately to find out what is stopping them completing their tasks and whether they need any help unblocking any barriers - e.g. their boss could be loading them up with other work that takes priority, in which case you need to discuss this with their boss to remove the blockage.
You might also have a discussion in the next meeting about any outstanding/overdue action points and, as a group, decide whether any need reallocating or redefining so that progress can be made.