Check the temperature recommendations from the filament manufacturer, as it’s most likely PET, what you are printing, the temperature should be around 240-250°C for the hotend and 60-70°C for the heated bed.
Also better use a little bit too much gluestick, then too less. I usually swipe the gluestick around the bed until i have a visible layer of glue applied, you’ll also feel, that the surface is sticky, but not wet, when touching the heated (!) bed. Disable your cooing fan or let it just run slow. PLA has a very low contracting factor and usually warping is also not that much of an issue with PET, but it still can happen. Please check all things I listed above and google around to find other’s, that also have warping issues with PET (or whatever UPET should be). Definitely read all information provided by the manufacturer, usually they provide you with guidlines for temperatures, speeds, cooling and so on. Lastly try YouTube to find general informations on how to prevent warping and maybe you’ll find a guide for printing PET.
Explaining all the necessary steps and listing all possible reasons would take too long for me, I can just give you those general tips and advises on how to find more information. Maybe someone else at 3dHubs can recommend guides, YouTubers and forums or maybe they could point you to a specific thread on TALK. @Gabriela3d you had a thread about 3d printing related YouTubers, didn’t you ? Maybe you could bring it up and John can find printing guides there.
Cheers,
Marius Breuer