Kitchen waste keeps coming back

[SafeT]

😕
The property is on a 3rd floor and there is the dishwasher and sink down the same waste pipe.
There is also an anti syphon which lets air int he system.
Worked well over last 4 years suddenly the sink drainage starts to fill the dishwasher and then the dishwasher sometimes starts to fill the sink.
When I press the air valve (small black plastic needle) water comes out and it drains perfectly for a while.

The drainage seems to work fine for a few minutes and then the back up happens.

I replaced the anti syphon part and still no joy.

Help please!!

Ps. the drain is not blocked, it seems as though air bubble is created in the pipework??

 

[WHPES]

Either the flexi waste at the back of the dishwasher has dropped down and lying on the floor (should be held up at the back of the dishwasher) or the main waste pipe is probably restricted further down.