OK. I would try testing your parts out as I do not see any issue with your formula
Try to just return a text field on the first condition to make sure that works. Place it on your detail section to validate it
if {productionData.Date} In LastFullMonth then "Add" else "Ignore"
Did you get the right values for each row?
If you did then try the next part seperate
{productionData.partsProducedLH} + {productionData.partsProducedRH}
Did it giove you the right values for all rows?