i did an inventory adjustment to change the cost of a serialized item but for some reason the cost is not being recalculated. Is it possible to force a recalculation

presume you have Inventory Costing Preferences (NS Help) set to run on a custom (i.e. daily) schedule. So this is probably already resolved. Having said that, you may be seeing the incorrect cost recalculation if you are adjusting out and in on the same day. This is because when the cost is recalculated for transactions/adjustments on the same day, positive transactions/adjustments are considered before negative transactions/adjustments. refer NS Help: System Cost of Goods Sold Adjustments

Thats exactly what happened