Mesh can't "keep up" with bone movement I have an issue with my model.
My mesh can't "keep up" with my bone movement. Whenever I rotate my bone, part of my mesh (window) moves along with the door, but quite "delayed". My windows are set up as a separate mesh, and parented to a bone via vertex groups. If I move my "root" bone.

I'm sorry if I sound vague and unclear, I'm just completely stuck and don't even know how else should I google this.

The glass part still follows the parent bone, but "falls behind", as if it is weighted improperly. But I have checked, my vertex group has correct name and fully weighted to the controlling bone. Location, rotation and scale are applied to all my objects, armature included.

My first thought was to just separate  the glass part that doesn't move from the rest of its glass mesh(all glass and transparent parts of my project are made as a separate mesh, for better looking textures) and just merge it with my door's mesh, but I really don't want to do that, since I already have the whole thing unwrapped and textured. Thus I really can't connect my glass vertices to my door frame, that will mess up my uv and textures.

I tried messing around with vertex groups, but to no avail. This glass triangle follows the rest of the model, but as soon as I try to "open" the door, it does this.

I've looked through similar posts regarding mesh deforming and rigging, and could not find anything close to my problem.

I uploaded my file to Google Drive, if you want to take a look at it yourself. Yes I'm using 2.7 since I want to finish this thing and then move to 2.8(I'm not comfortable to use it yet).

https://drive.google.com/open?id=1Fs7PxW8On4vDV90qjyXmqrfKxQqcbbPx

I hope you could help me, thank you in advance.
Clicking on the 'windows' object, changing to Weight Paint mode and clicking on each of the vertex groups in turn we can see that the small triangle of window is part of the 'door_fl' group in addition to the 'car_body' group (red highlighting means a weight of '1' is assigned):



The small part of the window that is 'lagging behind' needs to be solely part of the 'door_fl' group. Because it is also assigned to the 'car_body' group (and therefore also under the control of the 'car_body' bone), the 'door_fl' bone is not fully in control, which is why it is not exactly matching the movement of the bone. When vertices have weights of '1' in two groups, the bones connected to those groups share control of the vertices 50/50. 

To remedy this, simply select the vertices that make up that part of the window in edit mode, click the 'car_body' group in the Vertex Group panel of the Object Data tab in the Properties editor and click the Remove button:



The window will now be only in the 'door_fl' group, only be controlled by the 'door_fl' bone and will move correctly.