Thank you for the quick response. Looks like there is something else going on here. When I had tried the alignment earlier, I had included a total of 144 images of various exposure times (15 sec to 60 sec) with different filters (H Alpha, O3, S2). I am using ZWO1600 Mono Cooled cam. I had changed various settings and after your response I used the default settings and only 7 images were aligned.
To make sure that I am aligning 'similar' messages, I tried aligning with the following results. Please note that I have modified Max distortion and Descriptor Type
With only H Alpha images - 60 sec exposures - Max distortion 0.33 - Pentagon for similarity
===== StarAlignment: 18 succeeded, 12 failed, 0 skipped =====
With only H Alpha images - 60 sec exposures - Max distortion 0.33 - Triangle Similarity
===== StarAlignment: 28 succeeded, 2 failed, 0 skipped =====
With all files (H Alpha, O3 and S2) - Max distortion 0.50 - Triangle Similarity
===== StarAlignment: 19 succeeded, 132 failed, 0 skipped =====
To see why the number of aligned images is reduced when images with different filters were tried, I next tried alignment of only O3 images as follows:
With only O3 images - 15 sec exposure time - Max distortion 0.50 - Triangle Similarity
===== StarAlignment: 2 succeeded, 60 failed, 0 skipped =====
So, what's happening? Why number of aligned images is changing when images with different filters are included even though the reference image is the same?
When only H Alpha images are aligned, more images of H Alpha are aligned compared to when trying alignment with images taken through different filters. Why some of those H Alpha images are getting "non-aligned"? Am I missing something basic?
Also, I see that in many cases PI is able to find 3 to 5 star pairs but not the minimum of 6 required. Tried changing a few parameters to relax the alignment process but nothing seems to work.
Here is the link for four images taken with O3 filter which were part of the alignment process:
https://drive.google.com/open?id=1CZi59GstUSUOLdXAkxoGHFwKyt6h6zLl