Riise's two appearances since his arrival from Fulham have resulted in a3-2 defeat to Peterborough followed by a 1-0 reverse at Leeds.
But the 28-year-old believes both games were there for the taking and has urged his team-mates to find their shooting boots in time for the visit of Barnsley to Fratton Park on 15 October.
"I played for the first time against Peterborough - a game we should have won. That was a confusing game because the result did not reflect the performance," he told the club's website.
"We should have got more points from the last couple of games. I think we did enough to get points from both of them. Alright, we were bad in the first half at Leeds but credit to the lads for improving so much after the break.
"We've created lots of chances in both those games - that's a positive we can take. But what we have to make sure we do now is make the most of the chances that come our way."