Design rejected : quality issue , even though high resolution design

All my designs are created at a high resolution of either 4200x2800 or 5376x3584 pixels, and they are all at 300dpi, which is typically considered a high standard for digital printing. To further assure the quality, I’ve had samples printed through Spreadshirt, and the results were quite satisfactory, showcasing clear and crisp designs.
Yet every time I submit them to the marketplace they get rejected because spread shirt feels they blurred and pixelated .
I will address those 2 points:

  1. First all image design are done in hd , and I process them with Photoshop’s 'Enhance to add detail to an image, reducing pixelation and improving clarity.
  2. I also use sharpening tool to improve the look of my image design

While I acknowledge the imperative of maintaining high quality standards on platforms like Spreadshirt to avert customer dissatisfaction, I am beginning to speculate that the rejections may stem from factors beyond quality control. It appears conceivable that Spreadshirt, facing an inundation of new designer applications, might be implementing strategies to limit the influx of new contributors. This could involve the adoption of stringent acceptance criteria or other mechanisms to manage the burgeoning designer community on their platform.