I have a similar issue documented in "Force Click Not Working on Macbook" but none of the resolutions mentioned in the discussion solves my case.
Here are my findings so far:
1) It only worked on the first day
2) I have "Force click with one finger" set for "Look up & data detectors". It still doesn't work. I switched to "Tap with three fingers" and the look up works fine.
3) I tried Force Click on other functions (eg: calendar, quicktime, dock, etc) all are working fine. This indicates that hardware is good.
4) I upgraded OS X to 10.10.3 and it does not work still.
I am totally stuck now and appreciate for any suggestion on what might have gone wrong. Thanks!