Hi! Based on my experience, here are the possible reasons for bed warping issue:

1. Bed temperature needs to be accurate

2. If printing large items (or just long items) infill matters a lot. Less infill or the hexagon/honeycomb infill really really helps with warping. A lot of my warping came from trying to print large items at >60% infill. If you get good layer adhesion, there really should never be a need for such heavy infill.

3. An enclosure of some sort. The piece I put on the top of the printer seemed to have made the biggest difference.

4. Getting a good first layer. Getting your bed leveling right makes a big difference. You want that first layer to push into the bed a bit, but not so much that it distorts your layers. Set the distance between nozzle and bed to about 80% of your first layer height. I use feeler gauges. I heat everything up and let it soak for a bit. Then I place my nozzle directly over each screw and adjust that screw. Then I repeat and repeat until it stops changing. Then I confirm by moving the nozzle to the center and if adjustment is needed, I adjust all four screws by about the same amount. Then one more pass over the screws.

Let me know if this will work for you. Hope this helps!