join key of tabl not in record set

[John thomas]

i have a table orders details with a sub form displaying the order, i have a combo box to display the product from products table to place in the sub form products box .I keep getting the eror message join key of table details not in record set I have checked my relationships and for keys and there all there .What have i done wrong .and how can i correct this
Plaese keep answers easy to understand i am a absolute beginner