this is a common problem-check former threads for bubbles and you will find some good infor.
You might try cleaning the venturi, which needs to be done every 2-3 weeks on my Aqua Clear skimmer.
Get about a cup of very warm RO water-not salt-and place the venturi intake into the container of water. It sill siphon off all the water in the cup and will clean out any salt buildup or blockage at the venturi opening-this takes care of my bubble problem when it occurs. There is alway some fine tuning on these skimmers when you get them, but if yours has been working ok and you now have bubbles, this will probably clear it up.
Good luck.