vokera linea 24/28 looseing pressure?

My boiler has just started to loose pressure, it's not been used all over the summer months. the heating comes on the rads get quite hot, the thermostat appears to work comes on and off. But checking the pressure it falls below the recomend for use. No leaks found, and i have not bled the rads since they were put in. any idea's before i call in a service engineer, i have represured each time.