I am a consultant that does not fit neatly into one of the Business Persons categories that Canada uses to determine if I can enter Canada to consult.

I also applied for NEXUS and this came up during the application process. I specifically stated that I would fully disclose any time I was entering Canada to consult and that immigration could use its discretion to determine if I needed to purchase a work permit for that trip. I can not use the NEXUS lane to enter Canada as I must go through immigration and state my intentions to consult. If I am just visiting I can legally use the NEXUS lane. I also secure approval from Service Canada via an LMO (Labor Market Opinion) application to use at the border crossing.

I have on multiple occasions been stopped at the border and interrogated concerning my entering Canada. On two occasions my contracting company had not provided the correct paperwork and I developed a long record of "trying to enter Canada without proper authorization." In two additional cases I was turned back at the border and sent home and told to complete the never before necessary LMO. Even with these I was approved.

They have since tightened the requirements for entering Canada to work. The rules are similar but not exactly the same for Canadians working in the US. I understand that one of the rules that must be absolutely followed is that the program is for citizens of Canada and US not just legal residents.

To the OP, are you a Canadian citizen? It may be as simple as this.