I agree with the answers that suggest givining your planet a denser atmopshere. I will link to calculations suggesting that a breathable atmosphere could have up to about five times the sea level pressure of Earth's atmosphere - possibly much more.
You might also want to consider giving you planet a higher or lower surface gravity, whichever will tend increase the buoyancy of gases which are lighter than the atmosphere.
Of course every element or compound which might possibly be present in an atmosphere will become toxic if it is present in too great a concentration. Even Oxygen, vitally necessary for life, becomes toxic in excess amounts. So if you increase the concentration of every probable and improbable element and compound to the maximum that humans can tolerate, you will find an absolute upper limit to the density of atmosphere at the elevations where humans (and beings with the same requirements) live.
Habitable Planets for Man Stephen H. Dole, 1964, 2007, discusses the environmental requirements of humans.
And the 1964 edition is online:
On pages 13 to 19 Dole discusses the atmospheric requirements of Humans. On page 15 Dole states that the normal atmospheric pressure of oxygen at sea level on Earth is about 149 millimeters of Mercury. The overall sea level atmospheric pressure is 29.92 inches of Mercury, or 759.968 millimeters of Mercury.
On page 19 Dole says:
To summarize, then, the atmosphere of a habitable planet must contain oxygen with an inspired partial pressure of between 60 and 400 millimeters of mercury and carbon dioxide with a partial pressure roughly between 0.05 and 7 millimeters of mercury. In addiiton, the partial pressures of the inert gases must be below certain specified limits and other toxic gases must not be present in more than trace amounts. Some nitrogen must be present so that nitrogen in combined form can find its way into plants.
So carbon dioxide would be a very bad choice to increase atmospheric pressure. According to table 2 on page 16, the approximate upper pressure limits for nitrogen and argon would be about 2,330 and 1,220 milimeters of mercury. So a breathable atmosphere could possibly have a total of pressure of 3,950 milimeters of mercury of nitrogen, argon, and oxygen, which would be about 5.197 times the sea level pressure of Earth's atmosphere.
I note that Dole believes that helium and neon could be present at even greater pressures in a breathable atmosphere, but the upper limits are rather uncertain.
And of course newer research may have changed the safe limits of various atmospheric gases.