could be because (i) HIF1 is already blocked and Co2+ has no effect or (ii) it is already induced in your system and Co2+ does not increase it further.
To test this, I would try other cell line(s) .
As for hypoxic conditions, you may put your cells to a flask (regular cap), then pump some N2 real quick, close the cap and incubate the flask for a couple hours - it is normally enough to induce HIF but not yet lethal for the cells.
To establish hypoxia conditions in a low cost chamber, you can follow this research paper.
Article Engineering oxygen nanobubbles for the effective reversal of hypoxia
We used a very simple protocol to create hypoxia conditions as you can read in the paper. CoCl2 induces chemical changes and not real lack of oxygen, therefore, its a good indicator of HIF1a, but its not real lack of oxygen and cell breathing is normal.
Also, HIF-1a will express under normal conditions also. Especially if your cell confluence is high, (80% or more confluence), you are likely to have high values of HIF`1a.
You can refer to our another paper for HIF1a expression during in vivo conditions.
Article Effective delivery of mycophenolic acid by oxygen nanobubble...