In my experience you don't get called with offers of anything extra unless either the due date is fast approaching and nobody has taken them or they are in locations that they chronically have issues finding someone to perform. Feel free to counter their offer with a higher price that makes you comfortable. The bonus is generally not for prior good performance but to get the jobs off their 'to do' list and onto yours.
Meanwhile, Walesmaven's suggestion is indeed excellent advice. Add more companies to your repertoire with different jobs and the opportunity for better pay. This is not to suggest that you ditch your current relationship but rather that you give yourself more options. You can check with Jobslinger.com to see what companies have recently had jobs in your area. These are likely to be taken already but do give you some idea of what companies to register with next. And of course a link to a list of companies is at the bottom of each page of the forum.