Buy a 8-speed designated chain for a 8-speed cassette and there will be no problems. See if you can manually move the chain to these two cogs while rotating the wheel - if you have a bike repair stand, or get someone to hold the rear wheel off the ground so you can rotate the pedals. Sometimes it is a matter of checking for stiffness in the individual link attachment points which is going to be more of a problem with the smallest cogs where the radius is less for the chain.