the classical method is to (partly) integrate the issue of the firm in your research project in order to create a mutual interest. You could also reaffirm the research commitment to data confidentiality to create mutual trust and promote veracity... Besides you can also propose to send at the end of the study a summary of your results to participants as a reward..
If you touch upon sensitive issues of an organization, its quite obvious that they would hold themselves back while sharing/giving any information. However, what you can do is frame the objectives and questions which may not indicate as you are asking some trade secrets. If you ask about some special ingredient of a cook which makes him/her very special, you may not get the answer. Knowledge management fails there. You may just have to rephrase what you are asking with a different vantage point.