You can try adding a layer of moss on top of your mix to slow down the drying time. If you re pot you can add moss to the mix.

Are they small plants in small pots? Is the bark new? Sometimes new bark needs to be broken in before it absorbs water and holds on to the moisture. You can try using smaller pieces of bark. I grow mine in large bark and clay pots. I live in Southern California where it can get dry and hot. In those times I step up the watering. In any case, you'll need to adjust your watering to the medium and check for how long it takes to dry out.