How to get more honest feedback in stand ups?

The "stand up" format has the problem that you are airing your dirty laundry in front of everyone else, showing vulnerability, thus people have every reason to stay silent.

Sometimes standups just aren't the place. Because it all kinda depends on the work environment you're in. Are these devs surrounded by know it alls or genuine folks supporting each other?

If you want to try to make it work - try being the first person to say at your next standup "Hey I said I would get this done but I got blocked by this thing. Anyone have reference/documentation they can share so I can get unblocked?"

We're not all perfect so I know there's a part of the project that makes your head scratch or is blocking you. Come up with something to get the ball rolling. In the context of a standup its important everyone feels like they're in a safe environment. Anyways, just model the behavior and see what you get.

I know, I know - now you're the one in a vulnerable state. But that's why you're a leader. You do the things others are afraid to for the betterment of your team and the quality of your product.

I'm not saying this is right, but what I would personally do is talk to the developer(s) who are running slow and ask if they could use some assistance.

I wouldn't bother with trying to change their behavior in the standups at all -- but that's solely because in the places that I've worked that did this, standups were of no actual value. If the team is functioning well, then everyone already knows where everyone else is anyway, and real help is obtained and offered outside of meetings.