Bike mechanic needs some help!

I mainly work with cheap bikes with cup and cone hubs usually under $5 hubs

I really hope someone with experience can help me get this issue sorted as I'm experiencing it with all the bikes I work on, so here is the issue

• cup and cone seem to be in good condition with no/very little wear or roughness
• fresh grease, fresh balls
• perfectly set cone and I tighten locknut up to torque
• wheel spins freely, unobstructed, with 0 play for solid axles and a tiny play for qr axles which goes away when I teghten the skewer

A couple of days later I notice that there's some play and the wheel is rocking side to side ever so slightly, but upon checking the nuts, cone nut and lock nut are still very tight againest each other, nothing's loose, so, what gives?

I'm assuming that since the nuts are still tight, then the play might be due to wear of the cup, cone, or bearings

Any Ideas on how I can keep my customers hub play free long term?

Thanks 😊