You have the (+) input of the first opamp biased at the same voltage (+6V) as the power supply voltage so the outputs of all the opamps are saturated as high as they can go. Instead, bias it at half the supply voltage.
You have the value of R5 the same value as R6 so that it severely loads down R6. instead, the value of R5 should be MUCH higher than R6.
You are using an old-fashioned quad opamp that has a low input impedance, a low output swing and many other poor spec's. Use a modern one.
Look at this document about sinewave oscillators from Texas Instruments: