Cladding shrinkage ?

I contructed a workshop last September with some heavy duty pine T&G V cladding and all went well at first. After a few months though the cladding started to shrink.
I spent a day with silicone sealant filling in gaps but the cladding has continued to shrink.
We ordered the cladding (19mm thick) pretreated and when it turned up it was still wet from the treatment so we left it in a well ventilated area for a few weeks to acclimatise and dry out.
Its got to the point now where the tongues are almost out of the grooves on most of the planks. They were treated with a preservative after installation to further protect them and when they were installed they were butted up tight against each other.
I've spoken to the Manager of the timber merchants I purchased them from and unfortunately he is off on holiday for a couple of weeks but will be passing the issue on to a colleage to look into. Until a resolution has been arranged I'm not prepared to disclose the name of the merchants.
The question I've got is how much shrinkage would you reasonably expect from treated 19x125mm cladding ? I accept its going to shrink and expand according to the weather and climate but this has just shrunk and shrunk !!! Its now got to the point of not being weathertight anymore.
Thanks in advance Dom
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