sounds like the kite is too far forward into the wind window when launching. ( This can happen to me on the beach if you get launched by someone in a bit of a rush.)
The kite needs to be "full of wind" before your assistant releases it and you try to raise it. Is your assistant trained in kite surfing??
It's a similar condition to having overly long back (steering) lines or launching in a "wind shadow". ( is your kite in an area of potential turbulence, like behind a small dune? )
essentially it's all the same thing,! ! !
- you've not got your kite powered enough by wind, so it drops away looking for more. By the time it finds it, it has dropped sideways a little and tends to start rolling along. This can lead to a "hot launch" where your kite is now too far down wind and is super dangerous! ( Deadly - no exaggeration! Learn where your release is when you move to full size kite. Some guys launch with one hand on high side of bar and other hand on safety release ).
Your kite launching friend needs to walk to a point where the kite is full of wind and sits comfortably on the beach with no requirement to hold it at all!
This is the perfect launch point. The kite will just sit on the sand and wait for you to direct it slowly upwards.
If you can't find this point then you have overly long rear lines which can be shortened maybe 75mm at a time ie. change knot settings ( you can obviously alternatively lengthen the front lines with small extensions to achieve the same effect )
Really it's all the same reason for a kite to roll, but with many different causes. Work out what's wrong and fix it. You wont need a different bar, you may just have to tune that one. The shop guys should be able to sort this through with you.
Another reason is wind that is too light.
As you can realize, that's the same problem again! Not enough wind in your canopy!!
Don't give up, you'll get it!