Hand Shower Pressure

I have a new hand shower/shower bar installed in my shower. However, If I tighten the shower hose that connects to the to the wall (elbow fitting) so that no water spays out from that connection then flow from the shower head falls to a trickle.

However, if I losen that connection so that water sprays (a little) from the hose connection then water flow is excellent.

Is this normal? I would rather not have water spraying out from the hose wall connection.

p.s. the hose has a backflow preventer on it and is new (hansgrohe)

Could the backflow device be installed backwards?

By chance... is there a washer at this connection that has a small hole in it which more or less acts like a watersaver? If it has one of these type washers it could be that tightening causes the washer to close up thereby closing the flow of water to the shower head. You may have to use teflon tape or TFE to get a better seal at the connection.