Thanks for the replies guys, keep them coming.
This was cut from a 50mm square piece and I turned the fountain pen from the same piece.. So I agree oily could be the problem but I have always used white spirit on cocobolo and rosewoods, sanding sealer and then start with my CA.
I don't tape my own blanks I use rubber bands.. But can see where your coming from.
As per answer for Jim above, but agree it could be oil.
But the piece of wood came out of a piece of 50mm square timber for this and the other fountain pen. I can't see why the other one that was turned out of the same piece of wood and same technique has not currently shown the same problem. Unless that waiting to happen.. 
I might just decide to strip it down and see what it shows.. But then again..