Hi Tom–
Sometimes, problems with low notes (or any notes for that matter) can be caused by one or more of the following (in no particular order):
1) Blowing too hard. Try blowing more gently on the lowest notes.
2) Not closing one or more of the finger holes all the way. If the recorder has a separate bottom foot joint, try rotating it slightly to find the best position for your 4, 5, 6, & 7 fingers.
3) The way that the recorder was made if some of the low notes make a bubbling or burbly sound similar to a wolf on a bowed stringed instrument. Sometimes an alternate, non standard, fingering might help unless the problem is on the very lowest notes. At other times, there isn’t much you can do other than blowing very gently.
4) The windway and block surfaces might have some crud (the nicest word i could think of) or raised wood grain that disturbs the air flow. Try shining a light into the window and look through the windway from the beak end. If the windway looks rough, find an online tutorial that explains how to pop the block out and smooth the upper surface of the windway and the top surface of the block.
5) The head of the recorder might be cold and condensation could be forming which disturbs the airflow. Try warming the head for at least ten minutes so that it gets to about your body temperature before playing. Some people stick it in a pocket. When possible, I stick the entire recorder down the front of my shirt with the top of the beak against my chest.
6) One or more of the finger holes might be partially clogged with crud. Or, if wax has been used to fine tune the size and/or position of a hole, it could have changed shape or collected some crud. Try cleaning the suspicious looking hole or holes very carefully.
7) The recorder might be made to use German fingering instead of Baroque/English fingering. The wrong fingering will make some of the lower notes sound not as good as they should. Try to figure out which fingering system your recorder uses. The easiest way is to look at the relative sizes of finger holes 4 and 5. On German fingering recorders, hole 4 is larger than hole 5. On Baroque/English fingering recorders, hole 4 is smaller than hole 5. BTW, Baroque/English fingering is preferred by most all serious recorder players.
8) It is just possible that the reason that you were able to pick up the old recorder is that it was simply deemed to be less than satisfactory by its previous owner. In that case, one old standard joke popular among woodwind instrument players is that it might make a nice lamp body.
All the best.
–Dick–