Boiler not firing up properly

[am_diy]

currently having issue when turning my heating on. When turning the heating on the boiler does not fire up properly/fully so heating and water do not get hot. This seems to happen when the anti frost / pipe stat have already been running. Any ideas what the issue could be? When calling for heating the flame bar initially fires up before dropping right back down. Thanks.
Have attached phot of flame bar.

 

[CBW]

Is that a system boiler? Check the motorised valve if it is

 

[am_diy]

It is a system boiler. Have 2 zones for the heating with motorised valve for each zone. Both zones have the same issue. Only way I can seem to get it to fire up is turn the pipe stat down to the lowest setting, leave it for 30 mins to cool and then turn heating on again.

 

[CBW]

What temperature is it set to on the front?

 

[am_diy]

Flow is set to 75 degrees