It is about angles! We had that same harness before we upgraded. Anyway, tips:
1) If the reciever is squatting onto the dildo, have that person hold the dildo and aim it. Trying to just sit on it hands free is more difficult esp if you haven't warmed up. So at first, the squatter needs to grab the dildo as he/she lowers onto it. That person can feel what is going on and can aim better and angle better. Once it is in you can let go.
2) Warm up. Even though it is a small dildo it will still help to prepare your bum if you use the dildo maybe as a standalone first for a while.
3) Try different positions. You will find some easier than others. To the penetrator...you want to try and imagine you are kind of (but not so severe) aiming for his/her belly button. Like you want to touch the tip of this dildo against the inside of her/his bellybutton, lol sorry couldnt think how best to describe it but this is the angle you want to try and get, in almost all positions. So in doggy position, I like to push the dildo down with my hand until its justttt a little below pointing straight.....I think this makes sense?
4) Plentyyyyyy of lubricant
5) Patience. Whenever we have this trouble it is usually related to him not being relaxed or tensing up or just not quite ready, it happened a lot more at the beginning but we are just getting to the point now where its almost always easy. Let the receiver control the angle, speed of insertion etc until they are comfortable.
6) Our easiest positions are receiver on top, or reciever laying down with legs back and bum under a pillow as Aqualaria described.