I had similar reasons (e.g. avoiding bias) for not explaining reflection that much when I first started doing the reflection activities with my students.

What I ended up discovering was that without much explanation, many students treated the reflection as another course evaluation that they could fill up with complaints and issues they had with the course, and what should have been done differently. This was not an ideal outcome as the students basically got distracted from doing the reflection activity properly, which was supposed to help them discover and learn new things about their actions that they hadn't thought of before.

To address this problem, I have focused my explanations on telling the students to think more carefully about what they did during the activities, what challenges they faced, and what they would do differently in a similar situation. In this way, I didn't necessarily bias them or tell them what I wanted them to write, though I do always have concerns they are simply writing what they think I want them to write.

One other thing I noticed was timing was absolutely critical, and students had a much easier time reflecting right after a specific activity or project rather than if they had to do it after a few months.

I also made sure to give them a "satisfaction" survey separately around the same time as the reflection in which they could evaluate the activity and what should be done differently, and this also ensured they knew the reflection was different from a typical course/activity evaluation.

I'm wonder what was the percentage of students that did the reflection according to your guidelines and how many did not? I always have students who do not do the reflection properly no matter what instructions I give, but generally now, I have 70-80% of the students do reflection the right way. Its not so much about having all the students reflect, but at least ensuring that about 80% do the reflection the right way.

So if you notice an increase in the number of students doing the reflection that right way, that is big positive.

*Sorry for the long response, but I think this is a fascinating discussion on a very important learning activity*