Problems with varnishing a walnut stairs

Hi, I got a new stairs in recently with walnut threads. Im having problems varnishing it. To date I have sanded the threads, vacuumed them, wiped them with white spirit then varnished them with Ronseal Diamond Coat satin finish. I'm on the fourth coat now but I cannot get a smooth finish on the steps. I made doubly sure before the last coat that the step was sanded smooth and completely dust free before applying the varnish, but when dry the steps still had a gritty feel to them when you rub your hand across them.

I have been using a clean brush, and stirring the varnish well.

I'm at my wits end - any and all tips on what I might be doing wrong or how to fix this would be gratefully accepted.