This reference states that the control voltage input of the 555 in the astable mode should be no lower than 1.7V, for reason not stated. So, your control input would have to be level shifted/amplified, if that is true.
**broken link removed**
"The control voltage may be varied from 45 to 90% of the Vcc in the monostable mode, making it possible to control the width of the output pulse independently of RC. When it is used in the astable mode, the control voltage can be varied from 1.7V to the full Vcc. Varying the voltage in the astable mode will produce a frequency modulated (FM) output."