why does the prominant ion change from time to time?

I'm study some taxans with a Finnigan Lcq Advantagesysterm. The
prominent ion of some compounds changes from time to time. Sometimes
it's [M+H]+ ,sometime it's [M+Na]+. Do you know how to resolve this
problem? If I want to make sure that [M+Na]+wii be the prominent ion,
need I add something to the mobile phase, what can I add? Thank you
very much.

the is listed below
  
                                                  
flow rate (ml/min)                                       0.8
mobile phase component(ACN:Water)                        49:51
Spray Voltage(kv)                                        5
Capillary Temp(degree)                                   200 
Capillary Voltage(v)                                     5
Tube lens Offset(v)                                      30
Vaporizer Temp(degree)                                   480