Probation can certainly be revoked, if the sentencing court concludes that the probationer violated a term or condition of probation. Typical reasons for revocation include failing to pay restitution, failing to show up for counseling or other services, not performing community service assignments, not showing up for weekend jail days, continued drug use (as evidenced by tests), consorting with individuals you’ve been told to avoid (gang members, victims), and, of course, subsequent criminal activity. Note that as to the latter, revocation can occur even in the absence of a criminal conviction for that subsequent activity. Courts will appoint counsel for probationers facing probation revocation hearings.
Go to the Sentencing, Parole, and Probation FAQ.