Peppers are a bit picky. They need plenty of sun, but overexposure can scald the fruit. They need plenty of water but will suffer if the soil is too wet. They need consistently warm temperatures (over 50) day and night.
If they're in a shady spot, not getting enough water, or too much water, or are in poor soil, they won't perform.
You could try fertilizing them, that's about the only solution beyond watering more if you think they've been too dry.
Some years my peppers do great, but this year mine are stalled as well because our summer (in Wisconsin) came late and even now we've had some cool evenings recently.