Well, it’s probably a combination of both the recorder and your technique. The low notes on an alto recorder are difficult for a beginner. It takes lots of practice. When I first started to play I wanted to throw the recorder out the window. I don’t really find it to be much of an issue any more. This is what helped me:
1. the foot joint needs to be aligned properly; the pinky needs to reach the holes comfortably. There is a precise spot where that’s occurring. Once you find that spot you should remember where it is.
2. the breath pressure has to be just right. You can drop your jaw and blow slow, warm air
3. what helped me the most was to actually hear the pitch in my head before playing. When you do get the note in tune and with a full tone remember what it sounds like
4. play long tones; play low F for as long as you can hold it (you can use a metronome to time yourself).
I don’t know much about a Triebert recorder. It’s not available to buy where I live. Some recorders do have stronger bottom notes or stronger upper register than others. There are two alto plastic recorders that I have found to be very good in terms of intonation and good response in all registers and easy to play for beginners: the Yamaha 312 Biii and the Aulos 509 B. The Aulos Haka 709Bw is also very good but a little more difficult for beginners. The F is harder to get in the beginning, although once you get used to it it’s not really a problem. I hope this helps.
One problem I have noticed with most recorder methods is that they start with the notes that are easy to play. I personally started with the difficult notes and that paid off. Whenever I start playing I always start with low F and then play various intervals that involve low F. Low F is my “grounding” note.