I've recently received a tenor recorder, but after practicing for some time I find myself unable to reliably play middle C (Lowest possible note). The model is a Yamaha YRT-304B II, meaning it features an arched windway. Some searching told me that arched windways allow for easier playing in the higher registers.
Could this be the problem? Or should I focus on my fingering or breathing more?
It's hard to diagnose instrument playing problems over the internet. The easiest way to tell if the problem is with you or the instrument is to see if you can find a a somewhat moderately talented recorder player (one who has experience playing something other than a soprano) and ask them to play it.
The lowest notes on recorders, especially larger ones, can be notoriously difficult to get a clear crisp sound out of, due to the soft breath required. I see this model has a key, and that could be another contributing factor.
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.
The previous two answers cover it. I'll just add that the curved windway has nothing to do with the difficulty of playing the bottom C. The main reason the bottom note is hard to play on Baroque recorders (assuming the factors the previous answers mention are controlled) is the strong conical bore, which favors the higher registers. A good medieval tenor recorder will have a booming, easy to hit, low C, but at the expense of the higher notes.
