Blue Grid Transfer Paper Problems

Hello, I have a problem with dark transfer paper (Blue Grid). With heat press and medium pressure, I use 350 Degrees Fahrenheit and 10 seconds. sometimes I use variations of temperature: from 350 to 380 and time: from 5 to 20.

I have a T-shirt printing/transferring production workshop.

Almost always the image looks great after the heat press. great colors and great clarity/sharpness. Just fine.

But after a day or two, the image gets blurry - like a Photoshop filter - Gaussian Blur. It happens all the time. Always. The T-shirt just stays in the room, no washing, no extreme conditions, just normally lays on the table for a day. Sometimes two-three days, and the image gets blurry. Very blurry.

I don't know what's wrong, what am I doing wrong? The White T-shirt printing and transferring has no problems at all, but when I print for dark T-shirts, this happens.

Maybe the printer ink is low quality? but if it is low quality, then white T-shirts would have the same problem too, right? Maybe the "BlueGrid" paper is low quality? Please help.