Think about the sequence of things that must happen each time you start a note on the trumpet:
- Form your embouchure and place the mouthpiece.
- Breathe in through the corners of your mouth so your lungs inflate.
- Plant the tip of your tongue in the angle between hard palette and top teeth (not between your lips or behind your bottom teeth). This seals your mouth so no air can escape.
- Contract your chest, increasing the internal air pressure within your chest and mouth.
- Draw your tongue back sharply as though pronouncing the syllable "Tu!". Your tongue should then be flat in the bottom of your mouth. Don't let it get in the way of the air stream.
- The note starts immediately with a beautiful crisp attack.
- Follow through with your chest contraction so the wind speed stays constant.
The sequence is the important thing here. If you do step 4 without first doing step 3 then there will be no air to produce the note. If you tongue first and only then try to produce a stream of air the production will be unreliable.
Having read this you may already know what the problem is in your note production.
One other thing to do is to practise producing those low notes. Set your metronome going and then play 8th notes on a low C. The goal here is to internalize how it feels to produce those sounds. Good notes feel different to bad notes and you need to repeat often enough that the good happens more often than the bad. Do this for a few minutes a day for a couple of weeks and you'll hear the difference.
Acceptable consonants for starting the note are "Tu!", "Du!" and sometimes "Ku!" (if you're double-tonguing). But don't use "P" or "B" sounds because they disturb your lips.