Not enough air flow coming from the vents on the...

Question

I would like to increase the air flow on the second floor of this two story house as there is only a trickle of air coming through the vents. The temperature is a good 4°C to 5°C cooler in the upstairs rooms than on the ground floor level (this is in Mid January).I have checked the round disc plate inside the ducting on all the vents and they are all fully open. I have checked the loft insulation and that appears more than adequate. I just don't think that the air flow coming from the furnace is enough to reach the upper level of the house.I am a new owner of this 30 year old house and confused about the performance of the furnace which is a RUUD 90 PLUS ACHIEVER, it appears to be about 10 years old.