That model is a workhorse and should be capable of a decent, if not over-loud bottom note.
The most common problem is the right-hand. You have to cover all the holes completely, and press the key with the little finger (finger 7) correctly also so that all holes are covered. Finger 6 - the D/d# hole, is particularly sensitive for a leak.
The left-hand typically isn't a problem assuming G4 (T123) speaks easily.
Breath is the next most-common problem. As stated by the other answers, breath pressure must be rather low, but you can tongue the lowest note firmly with a "du" tongue.
Lastly, if the recorder itself needs maintenance then all the above isn't worth much. Verify the pads on the keys seal their holes correctly when the key is pressed (note there are usually 2 keys, one for C and one for C#. The C key is (IIRC) the top of the pair).
If you can make a good low D (T123456) but not the low C then perhaps you're shifting your right hand in order to press the key and therefore leaking 6. If that is the case, twist the bell to move the key into a more ergonomic position for your hand.
The key should not take a lot of force to hold down properly, assuming it's been maintained well.