Typically, there are 2 situations...
One is where there is a hole in the rear of the case, and the wrist pin needs to be removed from the piston / upper end of the rod before you can get the rod off the crankpin.

The other is typically caused by gum / varnish or rust on the wrist pin, keeping the upper end of the rod from sliding back far enough to get the rod to come off the crankpin.

Both can be fustrating.

The gum / varnish needs to be softened or removed. Acetone, carb cleaner, or some such solvent...then PB Blaster or penetrating oil, and heat can help to get the wrist pin free enough to slide the rod back. A zip tie around the upper end of the rod can give you some leverage to pull on it too. Work the upper end of the rod back and forth on the wrist pin to work in the solvents and lubricants.

The same sort of gunk can cause problems getting the wrist pin out through the hole in the rear of the case...(more typically found on 4 strokers )

It can take time...lots of wiggling and jiggling...some cursing (optional ) and patience...