How do you handle a PhD student whose thesis data collection keeps stalling?

I supervise a PhD student in exercise science whose data collection has stalled for months, partly due to recruitment problems and partly due to their own hesitation about approaching participants.

We have already adjusted the protocol once, but progress is still slow.

I am unsure whether to push for a timeline, suggest a different design, or focus on building their confidence first.

How have others handled a similar situation without damaging the supervisory relationship?