How tall and old and heavy is your child?
The Scenera Next doesn't require noodles or towels to keep the recline proper like the Guide does. It's much easier to install.
However, a tech's job is to teach YOU how to install your seat and make sure you're comfortable with it. If the techs at the police haven't done this, twice, then I wouldn't go back a third time.
For the Guide, until the Scenera Next arrives (and may negate the issue of replacing it all together), get behind the seat and as you tighten the lower anchor strap (if you're installing on the sides) or the locked seatbelt (for anywhere in the backseat), lean your hips into the seat. That will help the angle stay. There's nothing wrong with your seat, it's just a matter of getting your angle proper and keeping it there. Your manual suggests a rolled towel or pool noodle to help with this, and pushing it back and into the backseat helps a LOT. It's very nice of Dorel to send you a new seat, since there's nothing wrong with your old one.
Wendy