Most commonly, your SKU is missing a Pricefalls Category ID or another required attribute in Zentail. Here are Pricefalls required attributes, please make sure your SKU(s) in question have all required attributes.
** Please note Pricefalls accepts your full catalog once daily so SKUs listed after 11am PST will not show up until ~11am PST the following day. **
Here's how to view the data that is being sent to Pricefalls: