Karen,
 
#1  Make sure you are getting a good print.  If your using a inkjet/bubble jet make sure your heads are clean and not dragging.  (I would recommend using a laser or thermal printer)
 
#2  Did you change the font or size of the bar code?  If so, the lines could be spaced too close together and you will get false reads.
 
#3 What type of paper?  If you are using a light paper like an onion skin,  the laser will go right through it and cause problems.  Also if the paper has a sheen or gloss you will get false reads. (learned this the hard way when I laminated a list of employees with barcodes for DC-A login!)
 
#4  Clean the lense on the scanner.  If there is greese or a smear on the lense, you will get false reads.
 
#5  If it's a wedge scanner that goes on the KB line,  Check for spyware on the system (Recommend Ad-Aware).  Spyware can track and interrupt keystrokes  causing false reads.
 
#6  Could be the scanner or it's connection.
 
Hope it helps,
 
Tim