Unlike lambda, M13 is a non-lytic phage and does not produce clear plaques. M13 plaques are areas of diminished cell growth, not lysis, and consequently can be difficult to see. Try holding the plate up to a light. Also, since the vector used to prepare the library carries the lacZα gene, plaques will be blue, and easier to see, when using an alphacomplementing strain such as the supplied strain ER2738 and plating on Xgal/IPTG plates. Also, be sure the dilution range is appropriate for the phage you are titering. For amplified phage, plate 10 µL of 1:109 - 1:1011 dilutions; for unamplified panning eluates, try 1:10 - 1:104 dilutions for early rounds, 1:104-1:107 for later rounds. If the phage is not sufficiently dilute, the plaques will be confluent on the plate and it will look like there are no plaques at all (or a bluish tinge when using Xgal plates). Occasionally after PEG precipitation, the phage will clump and not dilute properly. As a result, you might have a plate containing too many plaques merged together. Make sure to give the phage ample time to resuspend after precipitation (> 1 hour) and vortex each dilution tube very well (~10 seconds).