Y - Axis Issue, set screw is tight

OK the activation of the y switch the plastic end piece the y belt is fastened to. As far as teh slippage is it possible you over tightened the y belt,

Over tightening will result in the exact same problem visually but caused by a completely different thing

The belt should make only a very low twang sound just like a guitar string does when plucked but it should be very very low. It needs to be just a bit more than the thud noise that is made when it is too loose.

When the belt is too tight it causes the motor to skip steps versus having the belt jump a tooth because it is too loose. The result of both things is the y axis is not moved when it should and a shift occurs in the print.

Hope this helps

If this is not the case the upload some pics so we can see what going on and we may be able to catch something else

 

Your bed may have mechanically shifted. The rails that are used for the y motion are dual sliding rails meaning they slide out in both directions from a central point. There is a bearing that itself rides in the lower rail and then the upper rail rides in the bearing now when this is in the true neutral postion meaning it does not extend in either direction that bearing should be centered in both the lower and upper rails allowing for full movement in both directions. That is the ideal state.

Full movement is when the bearing slides to one end of the lower rail and the upper rails slides in the bearing until it reaches the end. This allows for amore stable and longer movement while having shorter individual pieces of rail

Now when it mechanically shifts the bearing does not move for what ever reason maybe it needs slight lubrication or what ever but it will start to shift along one or both rails so that it actually stops moving because the bearing repostioned so it hits the end points sooner in one direction then in the other.

what you will see when this happens is the the bed does not have full movement back and forth and in your case does not contact the Y switch which it should.

The fix is to remove the bed surface from the rails and to manually reposition them so the bearing is in the right spot.

The first step is to move each rail back as far as they will go. They should be the same distance back if they are not continue to move the one that is more forward back by pushing the rail backward it is supposed to be hard to do so dont give up too easily you should be able to move it back to a point where they are both even.

Step 2 is to note how far back the rails go from the black central plate and then move both rails forward as far as they will go and again measure how far the ends extend away from the plate.

This distance should be the same. if it is not then you need to move both rails in the direction that is shorter so if the front is less then pull the rails forward if the back is less push the rails back. Either way extend them fully again and continue to move both rails so they move them the same amount as 1/2 the difference between the first two measurements.

For example if the rails extend 1 inch further back then they did forward slide them both fully forward and then keep moving them so they both are moved an additional 1/2 inch forward. This should center teh two rails and make the equal to each other as far as normal motion goes.

Replace the bed and you should now see that the piece that holds the front end of the y belt no makes contract with the y switch.

 

Yes, what tesseract said. I had this happen at one point. You need to tear down (he has detailed this well) the bed and reassemble making sure the rails are aligned the same on both sides.