Yes... most issues occur in these 3 critical areas.
Here's what to look for: (failure to meet these print file requirements will result in production delays)
1-Resolution
Your resolution should be at 300 dpi (dots per inch) at the exact size of your flyer. If your flyer is 4"x6", it should be sized as a 4"x6" at 300 dpi.
Often your computer program will default to 72 dpi, you must check the resolution of your file before uploading and that may require you to manually change the dpi to 300 from 72.
2- Size
Double check to see that the size you are submitting is the size of the product you are ordering.
For instance, if your flyer is 2.75"x4.25"... then that is the exact size your submitted file should be.
3- Bleed
Bleed is a printing term that refers to printing that extends to the edge of a sheet or page after trimming. To achieve that, for a 4" X 6" flyer... your image should also be 4" X 6".
However, keep text, borders and other images that cannot be trimmed at least 1/8" from all edges. This keeps your piece safe during the trimming process.