Find out why they are loose.
Is it really just these or is there a more widespread problem such as
drainage or a poorly compacted bedding layer? Testing the others
in the vicinity will show that.
You can begin by lifting them out carefully, plus the immediately
surrounding ones. Then assuming that the bed is solid and not a sea
of mud or other defect, lay in some fresh sand and tap it down firmly
with the end of a piece of wood whacked with a club hammer. Add more
sand and repeat if needed.
Lay in the blocks and tap these down with the club hammer using a
wooden block and a pad such as a piece of old carpet as protection.
When you are sure that the blocks are firm and you have a dry day with
the blocks dry as well, sprinkle some dry jointing sand into the gaps
and brush it well in. Repeat if required. Dryness is critical to
this step because it allows the sand to drop in the gaps and stop