I’m trying to have a prismatic constraint extend 2.3 studs in a time of 30 seconds, which means the Speed has to be set to 0.076. The constraint moves slightly until ~0.606 studs distance and then gets stuck there, and after testing I’ve noticed that this happens with any PrismaticConstraint when you set its speed below 0.4, making it impossible to reach the target angle at all.
Here’s a video example of the issue itself:
Have you tried to change the density of the blocks through the custom physical characteristics? or even make them massless?
Yes I’ve tried that, doesn’t solve the issue unfortunately.
Try making the force less than inf since I’ve seen that constraints sometimes act up when its that high.
Just tried it out, sadly still having the issue.
Hello!
I’m currently encountering a similar problem to yours and am in the process of looking for a solution on the forum!
I am using PrismaticConstraint to open and close train doors.
The train doors open and close slowly and I do not want the speed to be more than 1.
But as you can see in this video, if I open and close them too often, they stop in the middle and do not work consistently.
All parts connected to Constraint are Massless, have zero mass, and CanCollide is disabled.
The cause is probably, but not likely, that the speed is too slow and the parts are deemed not to be working and are put into sleep mode.
I think it is a Roblox glitch, but I highly doubt it will be fixed. 
Are you sure it’s not a collision issue?
Usually Meshes have different physical rendering than their physical rendering.
There is a setting in Studio Settings in the Physics tab called (I think) Show Collisions.
If the Meshes are colliding try using Collision Groups to keep the doors from hitting the cars.
It’s not a collision issue, as it happens with the simplest BaseParts with CanCollide set to false.
As already replied, all parts related to the door are set to be unaffected by physics. They do not collide with anything and do not carry any weight.
Sleeping very slow moving parts on their own is not a desirable behavior.
If you can fix it, that would be great!
@kleptonaut
im also having this issue, i use hinge constraints for a animatronic simulation and if the speed is set to low it gets stuck, I’m not quite sure how to fix this yet but hoping someday we figure it out
Constraints can have issues if the Density of the 2 connected Parts or Assemblies is different. For example having a Part that has a Density of 1 connected to another Part that has the Massless Property set to true will give weird physics behaviour.