It might be that your isofix points in your car are positioned low down and your back seats are quite sloped. This can cause a problem for rear facing car seats as it tilts them backwards. You can try some troubleshooting first:
First check the inserts are right for her age/size - there should be a guide in the book about what order to remove them, sometimes with a guide weight/height. 60cm and 6kg or 4 months is usually about right to remove the first piece if it's a multi stage insert or the whole insert if it's one piece.
Secondly check the shoulder straps are at the right height for her, as this can push them forwards if they are too low. Again the book will tell you how to change them if they can be changed. They should be level with the tops of her shoulders.
Third, when you sit her in the seat, many babies of this age are starting to sit up more and they want to sit up straight in the seat so they can look around. Don't be afraid to put your hands either side of her and reposition her, she should sit low in the seat with her nappy down to the buckle, in a lying position (not a sitting position) with her shoulders flush back into the seat. Don't worry if her back is bent (the insert can help with this). It can help to hold a toy above her, so that she lies back and reaches up for it. Otherwise they tend to lean forward and touch the buckle, their feet etc and if you tighten the straps in this position it pushes their shoulders forward out of the seat.
Lastly ensure straps are sufficiently tight, you shouldn't have any slack in them. If they are too loose they can also cause poor positioning.
If none of this helps then you can try to address the angle, by taking the seat off the base and fitting it with seatbelt instead. You might only need to do this for a few weeks until she grows a bit more and has a stronger neck.
If your seats do have a steep angle this is worth bearing in mind when you choose the next seat if you want to stay rear facing (which is definitely recommended) as some car seats are better than others for rear facing in cars with sloped seats. Isofix is often an issue.