Birdie4,
3 More 90's(at tank? 3/4 inch) and a horizontal run at the tank should not be a problem.
Any way to force flow? Like pressurizing the cold side?
Goal is to get hot water up to the tank, once.
How is the cold water added to the tank?
I strongly suspect air in the lines, even blockage. Assuming there are shutoffs in the system, check open!-)
If the firebox loop is always sloping up, then no place for air to accumulate...?
The water is hot up to the point it enters the wall and then up in the attic where it exits the wall and makes the first 90 it is no longer hot.
If none of above is the problem (air / blockage), must be a hot spot above the elbow in the wall. Very hot. Conduction can heat the water up to the elbow in the wall, not flow. Therefore loop flow is not getting started. A hotter spot is less dense, requires a driving force to bring it back down, if absent then loop not able to start.
If none of these work, will need pictures next! Heck send them anyways!
Have fun,
Scott.