A factory sealed iPad can be quite difficult to open. The iOpener does apply heat but not as effectively as a good electric heating pad. Personally I’ve never used the iOpener so I don’t want to speak out of turn. When I started repairing iPads, I used a MagicBag (hot compress) as that had the advantage of covering the whole iPad.
The trick to these bags is to heat it up, apply to the device and re-heat it again before it cools off. Try this a few times in a row on the same edge (for the iPad 6, the left edge) to get the maximum heat transfer. Then apply the suction cup and try to get your pick inside and work your way from there. Some techs use flat razor blades but I don’t recommend that for noobs (no offense ;-).
My personal favourite is the iFlex for getting in under the glass and then the iPlastix to slide it along as it doesn’t damage the glass. I use the iFixit picks to maintain the gap as I go around the glass.
You may want to check this After doing my repairs (battery and screen) I have problems for when you get to the closing and sealing of the iPad. There are quite a few gotcha’s that aren’t covered in the iFixit guides.
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