Asu,
I had this same issue on two units, I had to do my own research to get down to the bottom of this. To sum it up, I contacted Neutrik they make the combo XLR / 1/4 inch connector. They told me HOSA does not make all of their 1/4 phono plugs to a particular industry standard. For example, most 1/4 plugs tip is a one piece kind of smooth diamond shape. Now the HOSA in question; the tip is like a two piece triangle shape. So the straight edge bottom of the triangle is the part in which gets stuck by the clip inside the connector. I found that Pioneer had never heard of this (?) THANKS to my online vendor, they with out questions returned and replaced my 2 units. Yes, in happen to me twice until I found the answer to this issue. I have a youtube video regarding this, before I found this out. Peace.