EP0112640A1 - Process for obtaining high concentration argon by pressure-swing-adsorption - Google Patents
A process for obtaining high concentration argon from air by means of pressure-swing-adsorption, characterized by passing air through a zeolite molecular sieve-packed adsorption apparatus and a carbon molecular sieve-packed adsorption apparatus in this order, subjecting the air to...
Those two sources should get you started or at least point you in the direction of further research.
In a nut shell it’s Carbon Molecular Sieves not Zeolite that is able to separate Argon from Oxygen.
If your concentration is currently low perhaps you need to adjust the timing on your unit.