If you are talking about larch lap panels...I have sorted mine so they dont come apart...It can only be done when the wood is dry though... wait till the weather gets a bit warmer.
Make the necessary repairs, screwing the panels to hold them in place...
Get some Pink Grip... and inject the gaps between the the laps and the upright battens. When this sets, it will be harder than the wood...
Make sure you get Pink Grip...as this can be used externally... and it's really cheap... I pay about �2.50 for a tube...this should do a few panels.