How do I stop the wood on pre-primed canvas stretchers from buckling?

I don’t have the equipment to make my own stretchers so the easiest option is to buy the pre-primed canvases online. The wood keeps buckling after painting on the canvas so the stretcher doesn’t lie flat against the wall. Any suggestions of how I can prevent this or websites that sell reliable canvases at decent prices?

Decent stretchers will have mitered corners w/ interlocking "splines" & recesses for wedges.
The ones you are getting may not be the best built & the pre primed canvas may have been stretched "dry" & does not have a taut shrink fit.
When you paint ( acrylics ?) the canvas will first loosen then shrink from the moisture in the paint causing the stretchers to twist.
Common to tack on tempered masonite "gussets" on the corners.
Would put them on before painting.
The canvas will wrinkle if you "untwist" after it has been painted
Tacking into a strong frame helps .
A few ways to go.
Buy better pre primed & shrunk canvasses
Buy decent stretchers & make your own.
Not that difficult & you can save money .
Would try a plumbing supply shop & ask if they have muslin pipe wrapping.
Rarely used anymore & if they have it buy the roll.
It is tightly woven & as good or better than unprimed roll canvas from an art store.
You will have to prime it but many people use latex wall primer at a fraction of the cost of canvas primer & is basically the same thing.
If the "canvas " is stretched just enough to take wrinkles out the primer will shrink it about right .
Could also skip canvas entirely & paint on tempered masonite panels which is commonly done for oils or acrylics.
Still have to prime , front & back usint the wall primer.
Think you have been getting what you paid for.
Best regards