Innovus: Does not detect Inverters in library

Dear all,

I'm using Innovus to P&R a design consisting of custom made cells. The cells have been characterized and compiled into a .lib file using Synopsys SiliconSmart.
Everything works as expected, but I cannot do clock synthesis as Innovus does not detect my inverters as usable;

Total number of combinational cells: 16
Total number of sequential cells: 2
Total number of tristate cells: 0
Total number of level shifter cells: 2
Total number of power gating cells: 0
Total number of isolation cells: 0
Total number of power switch cells: 0
Total number of pulse generator cells: 0
Total number of always on buffers: 0
Total number of retention cells: 0
List of usable buffers:
Total number of usable buffers: 0
List of unusable buffers:
Total number of unusable buffers: 0
List of usable inverters:
Total number of usable inverters: 0
List of unusable inverters:
Total number of unusable inverters: 0
List of identified usable delay cells:
Total number of identified usable delay cells: 0
List of identified unusable delay cells:
Total number of identified unusable delay cells: 0

My .lib file does not have cell footprints, according to the Innovus user guide this should be fine as it figures out the footprint automatically, depending on functionality.
This is the output from 'reportFootPrint':

# footPrint: INVX1 nrCell: 3 Library: <hidden_bc>
Library:<hidden>
INVX1 INVX1 1 66.039 64.064
Y=(!X)
INVX2 INVX1 2 32.803 32.654
Y=(!X)
INVX4 INVX1 3 16.598 16.955
Y=(!X)
# footPrint: INVX1 nrCell: 3 Library: <hidden_wc>
Library:<hidden>
INVX1 INVX1 1 130.344 127.401
Y=(!X)
INVX2 INVX1 2 64.316 65.580
Y=(!X)
INVX4 INVX1 3 32.618 34.204
Y=(!X)

Clearly, Innovus also sees that indeed it is an inverter and detects all inverters having the same footprint.
Finally, I checked the 'dont_use' property, which is false.

Hence the question, how can I further debug this, or even better, what would be the solution?
(I am not allowed to upload the .lib files)

Thanks.