do i need to have my meds adjusted

I am a 60 yr old male. i have taken 20mg prozac for about 10 yrs. with good results. lately i have been more anxious and had a general sense of not being as happy. does an antidepressant like prozac lose its effectiveness over time. woudl it be helpful to try another type. i have also noticed a declining libido over time which may be nothing more than ageing. any suggestions or advice would be appreciated. this has not reached severity but am concerned at lack of general happiness. have not encountered any unusual stresses lately and life, marriage, relationships are good.