The following causes can be considered.
Solutions
Use originals supported by the ADF.
ADF Specifications
Load originals in the correct direction, and slide the ADF edge guides against the edges of the originals.
Do not load originals above the line indicated by the triangle symbol on the ADF.
Clean the inside of the ADF.
Cleaning the ADF
On the screen for copy or scan, check that the ADF icon is on. If it is off, place the originals again.