I had a similar problem last week in which I also tried both slot mates and path mates in motion with no luck. At one point you might be given the option of using a path mate but only if you lock it to a fixed point, which is not useful.
If you were not worried about where the endpoints were (like your travel is limited by the 'motor' movement), you could simply apply a coincident mate between the point at the center of your lug to the line I see in your path. This gives fast and reliable solve times but the lug can travel beyond the visible extents of the parts as defined by the slot.
If you want the part to stop when it reaches the ends of the slot, then the only way I know to do this is to use contacts. This is what I have done in my assemblies, and it works surprisingly well. In your case you would not be able to simply put the contact in alone, the parts would need to be be supported by other mates because otherwise it would be too sloppy. For instance, you would need to keep the faces parallel and you could probably still do the coincident mate between the point and your path line.
If someone else with more knowledge wishes to chime in please don't be deterred by my having responded.
Best of luck....
Thanks! That worked. I needed to limit the range of motion, so I had to use all 3 mates. Parallel, point-line coincident, and contact between the two parts. I hadn't used those 3 in combination before. I had thought parallel and contact mates would be enough.
I suspect you're running into the fact that not all mates translate into motion. sometimes you have to use primitives like you did. check the swx kb as there is a list of supported mates or articles indicating if mates aren't supported. sometimes you also need to go with contact.