I don't know if Im just being a little stupid or if this has already been addressed, but when I try and use a mine cart assembler the mine cart will flow straight though. not assembling the cart. Ive tried this on two different laptops and my server and all three times it does not assemble. it also that compared to videos I have watched that the assembler has smile pre applied.
Im playing on 16.4 with forge if that helps

Did you power the assembler with redstone?

tried It with and without and it didn't do anything

Please attach a screenshot

I have attached these screenshots. I think its a little confusing to show but the Minecrart just goes straight though the cart assembler.

This configuration worked only in 0.2.3 and earlier (1.14 versions of create). In modern Create you need to attach the contraption to the top (sticky side) of the assembler and give the cart assembler redstone power. This change was made along the other cart assembler changes as carts kept assembling their redstone power source and therefore assembled and disassembled rapidly, with undefined outcome.