Small changes in context and expectation can have large effects
The effect of assessment is very sensitive to the local context - of the course and its place in the course, of the student and their prior experience and current expectations. This means that several iterations of any assessment practice are normally needed to realise the potential of the strategy adopted. Often small changes in proceedure or communication can have a major impact of how it is received. Time spent in helping establish student expectations can often be beneficial.
Assessment designs often need to go through several cycles of modification to be most effective
Because students responses to assessment tasks are influenced by what they bring and the context in which they operate, it is rare that even superior assessment tasks can be designed once and then left intact. Assessment tasks will usually need to be refined over the first two uses and refreshed later as courses change. Re-design is not a sign of failure, but a sign of diligence and commitment to assessment process.