how can I fix my vokera compact HE35?

when I switch on green led appears, boiler will not ignite after a period of time red led will come on. If you remove the air cover box or slightly hold out at one of the bottom corners after remove of clip ! its all systems go. after securing the air cover box the boiler will operate normal; until you turn on your hot water along with central heating system boiler then will go into shut-down. On previous occasions when appliance has acted as above I have slip of the two rubber tubes that are connected to the air pressure switch which has resulted in appliance operating normal for both central heating and hot water