Try extending the doublet gate to the top of the graph. The size of the cells you want to look at have a bigger Fsc than your drawing your doublet gate for. It does look like many of the monocytes are in doublets but there should be some in the diagonal. You also might find it easier to use width vs height since you can draw a box and extend it to the axes. You can also gate on the live cells then doublet exclude which can sometime change the angle that you end up drawing your doublet gate at. I am concerned that much of the population you want to look at is way at the edge and against the axes. The next flow you run, I would adjust the FSC and SSC so that there aren't as many cells against the axes. This will also make it easier to identify the monocyte population.

Probably you are applying an extremely narrow FSC-A vs H close to linear identity function, which is proper for lymphocytes due to its almost spherical morphology. Monocytes are larger cells prone to deformation under the FACS hydraulic system, so you must include events with relatively low height (FSC-H proportional to major diameter) respect the area (FSC-A, the integral [i.e. AUC] H vs W). So you should exclude only extreme outliers. Probably you're being too restrictive. It might be useful to look at other aberrant markers for a monocyte (like expression of CD3, CD19 or CD15, or lack of CD45) in those cells excluded by your original gate (if those cells are really asymmetric monocytes and not doublets, then no bizarre markers should be found). Finally, it is important to keep in mind the current concepts in terms of CD14hiCD16lo vs CD14intCD16hi in monocyte subsets (this also depends on the stimulation you're providing).

Kind regards 

Hello Ming

I advise you check your cell detachment  treatment

Regards