Seems like a breath control issue, but could also be a poor instrument.
But there are many critical aspects here, particularly if you do not already have a music background. For example, you need to be aware of the phrasing and accordingly the continous flow of air needed to achieve this. As with the breath control needed for the lowest tones, this is a fundamental aspect of playing the recorder. If possible, a few lessons from a good teacher would be very helpful at this stage.
