Hi Stephen
what I was alluding to was whether or not the particular insert you are using able to cut the threads per inch you attempted.
I am (almost) confident that inserts have a range of threads per inch they can cut before you need to put a different one in the holder for the next range of threads per inch.
Whether that is 55 deg's or 60 degree's is only a measure of the thread angle and not the amount of threads there is per inch of shank length.
I apologise but I am possibly the worst at explaining things.
Phil
ps after a lifetime of machining and literally hundreds of threads I used an insert to cut a thread maybe 2 months ago. I have only ever used HSS (high speed steel)