What’s the scale value of your object? If the values aren’t 1,1,1 (X,Y,Z) then a lot of Blender’s tools won’t function as expected since they behave relatively to the scale of the object.
If your scale values are different from 1,1,1 then hit CTRL +A in object mode to bring up the Apply operation and choose Scale. You will see your object stay the same size but the values will be set to 1,1,1. Once you do that, go back to Edit Mode and retry thickening the planks.
