Cut Mortise not running in the center of my birch plywood template

First welcome. Frustrating. It would really help to have a couple of pictures of your setup, your jig and maybe the workpiece. Hard to tell from your description. If you take pictures, load them onto your computer and then you can just drag and drop them into your post. They go in wherever you have the cursor.

You see the line of icons just below the text entry box? Three icons to the right of the paper clip is a rectangle that represents insert a picture. (not the camera). Click on that icon and a popup appears. Have your file explorer open and drag and drop that picture onto the popup and it will insert the picture or pictures. It really helps.

One small thing that comes to mind is that you may have turned the router to the left or right, and not all router bases are the same diameter north south, as they are east west. That could account for the error. If you had to re-center the base, did you tighten it up sufficiently each time? Have you used a centering pin to center the baseplate itself before centering the bit in the bushing?

This could be a number of things, and a picture or two would really help figure it out.

Thanks much for your fast reply. First, let me mention that this is my first try making mortises with a router. My preference is cutting them by hand but the plans I'm using call for all mortises be made using templates. Anyway you'll see from the photos that I'm an amateur. While the mortise looks crude, if I place the metal router template in the wooden template there is hardly any play. I'm sure you'll have more questions which I'll be glad to answer. Again, thanks for your interest and time. MAC

Two more pics. Thanks. Michael

Thanks again for your suggestions. Terry Q. I haven't tried this yet but will. I decided to start from scratch and the results are better. A made a new template after sharpening the mortise and chisel. With my plunge router I made several shallow cuts rather than cutting the final 3/4 " depth all at once. You'll see from the attached picture, for this trial mortise I have it locked in my bench and the template is screwed to the maple base. The mortise is still slightly off but I guess I'm going to have to live with ~ 1/64" discrepancy. I wish the inside faces of the plywood mortise were less jagged but maybe they'll improve with making more of them. Again, thank you all for your interest, your help saved the day. Regards.

It looks like your router base is not fully supported since you jig is close to the edge of the template stock. I have successively used this technique on many projects. How did you make the template? I make my templates using the router table with a fence and get very good results. I use floating tenons so I make mortises on both mating pieces. This approach eliminates the need for squaring the ends.