sounds like a problem with pollination in a dry glasshouse environment. Misting your plants before they flower will help pollination and also help to control Red Spider mite which can destroy your plants.
If you're not already using a liquid feed, i would start feeding using Tomato food every 2 weeks when the fruits are setting.
Some people do pinch out the growing point as the season progresses. I think it depends on the variety grown. I'm sure the dwarf varieties don't need pinching