Originally Posted by GRUNTERSDAD
If these are your first double pedals my guess would be a weaker left foot. Try playing some hand rudiments slowly with your feet concentrating on technique and not speed. The speed will come.
Indeed, even though I have played double pedals before, these are my first pedals. My former experiences were just short bursts, not playing constant 16th notes or stuff like that. I`ve noticed I can`t go faster than 8th notes with my left foot, so practicing rudiments with with my feet should help. Also, I started playing various grooves with my left foot, to strenghten it. Pedal settings are even,both spring tension and pedal height, so problem solved :)