I have watched the video and checked the photos. Thank you very much for that. This is making things much easier.
First off all, the result is coming from the printer.
I am sure, if you check the print-out from the printer, you will see that the images looks more or less the same like on the shirt later.
Usually we recommend to use Foil/Film/Transparency/Glossy and not Heavy Paper, but if your result and fixation of the printer is fine, it is also ok. But I think something is happening with the file in the printer.
It is not a low resolution of the file but maybe some problem with the pritner and its resolution.
There is nothing we can do to help you infortunately.
There is only one thing for the future. If you have problems with the transfer, remove the teflon from your heat press and never print A sheet on top.
Always A on the heat press and the smaller B-Paper on top. In this case you just have to cover the sheet from the top to avoid that the glue from the B-Paper edges is coming to the upper plate of the heat press.