Duplicant keeps cutting toilet visit short

The problem is that the pipe is not empty, toilets can't be used when the "pipe cell" it is connected to isn't empty:

 

As a workaround I would suggest something like this, it is self sustaining, too:

@Azunai333 is correct. You can't have 2 devices drain into the same pipe within 3 tiles as that will clog the pipe.
If you space your toilets 2 tiles or route your pipes to be 2 tiles longer the problem you are experiencing will be fixed.