Uneven pressure or temperature
Uneven pressure or temperature can result in poor image transfers regardless of the type of heat transfer paper used. Heat press machines over time decrease in performance and it is a fairly common problem with older heat presses to have cold spots or to no longer apply the pressure that the read out says (or even if you go by feel). Check with your manufacturer to see what re-calibration steps you can take to tune up your press.
Some papers require really heavy pressure that only higher end commercial presses can provide, and pressure is a common problem with small hobby presses.
Double check and follow exact instructions
Every paper is different and requires different amounts of time and temperature and various peeling methods. Laser papers that are self weeding may call for a warm peel, which means waiting a specific amount of time before peeling. Some papers even require rubbing the back of the paper prior to peeling for best results. Double check the instructions that came with your paper.
Heat transfer paper does have an expiration date. If it is stored in a cool, low humidity place, the shelf life of every paper is about one year. Anything past one year from date of purchase would be considered expired and will need to be replaced if it causes any transfer problems that can't be resolved.
Color compatibility with self weeding papers
Some papers are better suited for photographs while others are great for detailed, bright designs. Laser transfer papers that are self weeding and are printed through a regular CMYK printer for example will not be able to transfer white or very light shades simply because the printer doesn't print white and the light areas of your design are not dense enough. A non self-weeding paper would be better suited for designs with white or light areas, or upgrading to a OKI white toner printers.