Raised bed problem

A few months ago I made a 9" raised bed solely with bags of compost and am now having a problem with it drying out then getting it wet.
It won't get wet all the way down, I've tried hoeing, watering then coming back later but it still puddles on top. Scrape away the top and it is very dry underneath.
It's full of veg at the moment but what can I do to prevent this happening again, should I add some soil or around here there is always some horse manure for sale, would this help?
I'd appreciate any advice.
Thanks
Rob

 

@RobB That is a problem with multi purpose, rewetting. Some contain wetting agents to help. At the moment a day or so of gentle rain would be best and then keep it damp. If it is a fairly small raised bed you might try using a watering can and adding a few drops of washing up liquid to each can full. Otherwise I think you have to wait until the veg is done and then dig some soil into the mix.
Horse manure, depending on the amount of straw etc can be difficult to wet as well. I would always get well rotted horse manure, compost it myself or use it as a mulch over winter.
For large pots I mix soil and MPC about 50:50 to avoid the rewetting issue, also makes the pots heavier, an advantage with plastic ones less so with terracotta pots.

 

Thanks NigelJ and JWK.
Grass cuttings I can get when I cut the grass, good idea.
The rain overnight, despite forecasts was quite light so still not wet but more to come later.
Plenty of soil on a bank at the side of the garden so when the crops are gone I'll get digging.
I did think of washing up liquid but thought ok for cement but on compost???
I'll get this wet spell over and make sure it doesn't dry out again.
Thanks again