Plastic recorders are fairly indestructible, but maybe it could do with a quick clean inside. Use warm water and washing up liquid and use a soft cloth in the cleaning stick to wipe out the insides. Give the head piece a good blast under the tap.
Then make sure all the joints are pushed fully together and the recorder is well warmed up. Your problem might be as simple as the change in season since you last played changing how much warming is needed.
Finally, some notes need more puff than others to play in tune and this is something you will learn from experience.
Descant and treble recorders.
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