Mark
Have you tried doing the mosaic separately with each channel and then combining them? I find frame Adaptation in Star Alignment (with 2-D Surface Spline Registration Model and Distortion Correction) has a better success rate on the individual channels.
Some mosaics just don't want to play ball using Star Alignment to build the composition in which case Image Solver and Mosaic By Co-Ordinates followed by Gradient Merge Mosaic can sometimes give a better result.
Chris