The bubbles like this can come from two directions. One is that you are not topping off the injection port with plastic. Some baits will suck plastic out of the sprue as they cool and if no extra plastic is there to take in the action will pull in air, hence the bubble. The sprue extender that's mentioned can help here by providing a little extra column of plastic but I'd try leaving the injector right in the port with some steady, but light pressure for ten seconds and then adding a small pile atop the port when the injector is removed. If you are adding sufficient plastic to the mold and know that air is not getting pulled into the cavities as they cool, chances are that you are getting air into the injector during the filling.

If you are pouring your plastic into the injector tube with the nozzle off, you need to keep the injector straight up after putting the nozzle back on and slowly pushing out a small amount of plastic to vent any air as it will be immediately under the nozzle. If you tip the injector down to squeeze that plastic back into a cup the air inside simply goes up to the plunger end of the plastic inside and will come out last and in the ends of the baits.

I fill my injector by putting the nozzle about 1/2" down into the plastic, pull up the rod about 1/8", then squirt that tiny bit of plastic right back in the cup keeping the nozzle submerged then pull up the plastic I need to fill the mold[s]. By doing it this way I can keep the injector straight up and down from cup to mold[s]. Any air that might be in the injector gets blow out when I push the small amount of plastic taken up before I actually fill the injector. Doing this leaves a tiny plug of plastic on the end of the plunger taking up the small air space found in there when the injector is empty so no air can re-enter when I pull up plastic to fill the injector for injecting. Filling this way also leaves a ring of plastic ON the nozzle where it fits the mold and this acts as a seal so air cannot enter or get sucked in when I do the actual injection into the mold.

I've been doing it this way since day one and I have very few issues with air getting into the baits.