Mirror not correctly mirroring object?

The mesh I am am making is complicated for me, so I decided to break it down into different parts as I modeled it.

The original mesh I started working with mirrored fine as I started with a plane, divided it in two, and then mirrored the deleted half.

Now when I modeled a separate object to piece together with the mesh afterwards, it does not mirror the same.

I tried making them have the same origin but it still mirrors incorrectly in all axes.

Can somebody help? Here are some screenshots.

Try resetting the Rotation & Scale by pressing Ctrl & A and then select Rotation & Scale from the menu. I think you do this in Object mode.

Thank you, it worked!