Hi there @Greybarn,
I think it might be because there's an extranious surface trapped in one of the stitched groups......that is probably the cause the failure to stitch to a final solid.
Actually, it looks like a self intersecting surface remnant, from the "Extend 4"....maybe mod that corner before doing the extend will help.
Design Manager Streetscape Limited
Solid Edge ST10 [MP2] Classic [x2 seats]
I will check those things. The strange part was that as soon as unlinked the part copy, I could turn it into a solid.
I will report back on that result.
Thanks for the help.
Have a great weekend.