May be a case of a chain that is too long and a de-tensioned derailleur... if it is working fine through the rest of the range it is probably this.
The small / small combination is the point where chain tension is the lowest and is usually considered to be a less than optimal gear choice because of the high degree of cross chaining... on a triple you will find the same gear off the middle ring and middle of the cassette / freewheel and enjoy a better chain line and proper tension on the derailleur.
Chain should be measured by removing it from the derailleur and wrapping it around the big chain ring and biggest rear cog and adding a pair of links... the b screw on the derailleur may also need to be adjusted to dial things in to provide adequate tension throughout the entire operating range.