Ouch! Ouch! Ouch!
The reason for the mistake was that the silk, being somewhat thin, has stretched and the satin stitches became looser as I progressed, forming a bubble. So I had to re-stretch the piece before starting. The moral of the story is - always check for tautness before starting.
I also prefer the tail fin to be more delicate; so that instead of using all 16 sugas I will be using 12.