Dull Colors when exporting to PNG for T-Shirts

I'm so frustrated and I've talked to about 10 Coreldraw Techs and not one person can help me!!  I have Coreldraw x8 and create my Art designs in CMYK.....then I export them to PNG...& put them on T-Shirts in my Online Web Store. 

I export them as a PNG (so I won't have a big white box background around my designs)...but the colors when printed on the T-Shirts become"Dull" and "Washed Out"!! :0((  

Other people who have stores where I do...have T-Shirts that look great and they have bright colors.  I don't know what I'm doing wrong??  Any help or suggestions would be deeply appreciated!! 

Thank you...Kerry