It sounds like you're working each shoulder separately. BO 4 st at the armhole edge, then do the dec when there's 10 sts left and p1 k1 for 8 sts (not 8 times). Then turn and work a WS row, and dec the same on the next RS row - work to the last 10 sts - k2 tog, p1 k1 for 8 sts. when you've done 4 dec rows, then switch to doing the dec every 4th row instead of every other row 11 time.
sue- knitting heretic