Cycloconverter , MOC3021 problems?
You need to exceed certain AC voltages on the switching side for this to work properly. Once triggered the triac will have an on-state voltage. For the MOC3021 this is typically 1.4 volts but can be as high as 3 volts. However, the AC waveform needs to exceed this by some margin to get the device to trigger. I think, if it's working on positive cycles, you are close to making it work on negative cycles too. Maybe try lowering the load resistor a bit and see if this begins to happen.

BTW what is your diode forward current? Your question doesn't specify this.