I had the same problem. I went to a fireplace store and bought a pre mixed mortor and filled it.However its starting to show again.Keep in mind I make a fire about 100 days a year and like them hot.So if you use the fireplace sparingly this might hold up better. They also told me that some fire boxes have three removable sections that can be removed and re ordered.I checked mine and they are held in place by screws and tabs.I think this summer I 'll order the back section.

thanks winterman. I'll check it out.

My mother's insert has replaceable firebrick on both sides and across the back. As I recall they are not full size bricks and are meant to insulate the steel sides/back from the coals.

If you could find out what brand and model your insert is, you can probably do a web search and find the manufacturers website. They should have an illustrated parts list available.