Solid mask not sticking to its keyframe

[slumberviz]

So i was trying to track my phone screen by using a solid, masking and keyframing it,

but somehow the mask doesn't remain the same when i go back few frames. please can anyone tell me how to fix this.step 1step 2result

[Rick Gerard]

Are you sure the layers have not moved? I usually add this expression to layers that I am going to rotoscope. 

[width,  height] / 2

This will keep layers centered in the middle of the comp. 

[slumberviz]

Hello sir. I am sorry i didn't get completely what you are trying to say, actually i am new to this software, so can you tell me how to use this particular expression while rotoscoping by giving an example stepwise?