Could dimerization account for weak IR signals?
Intensity is (in general) proportional to sample concentration, so yes, if you are performing IR in solution, then an overly dilute sample would have less intense peaks. But solution IR is somewhat uncommon; I suspect you are either doing IR on your sample in neat form (using an ATR stage) or in a pressed KBr plate? If the latter, you may not have used enough sample. 

Regarding your missing peak: sometimes peak intensities can change, depending on your sample prep. For example, if the peak in question corresponds to proton capable of hydrogen bonding (e.g., -COO-H, -NR2-H, -O-H, etc.) then the sharpness and intensity of the peak will depend on the extent of H-bonding your sample, which, in turn, can depend on things like sample concentration, the presence of water, etc. Sometimes these peaks get broadened to the point of being invisible if your sample is not very concentrated.

If your missing peak corresponds to a C-H, C-C, C-O, C-N, or some other non-labile bond, then there really isn't an explanation other than your compound not being what you think it is...