Baxi Wall Heater problems?

Can anyone help please. I have 2 baxi F5S balanced flue wall heaters and one F8ST heater. Since new, lighting all 3 of them has been really difficult. The main issue is that after the spark causes the fire to light you can hold the gas control knob in for far longer than 20secs and when you let go the flame goes out. This has happened on all 3 fires since new. The installing Corgi guy couldnt get one to light very easily at all and it was replaced by baxi after they changed lots of bits but was only slightly better. Once alight the fires are all fine and have never gone out. I have had Baxi fires for many years and my old ones (same as current models but without the flame failure device) work fine. Its now driving my wife nuts as about half the time they behave as above. I notice that when that happens its no good trying again for a couple of minutes; the spark ignites the gas and goes out instantly. I am guessing the correct air/gas mixtures hasnt been reached. Has anyone come across this problem as the corgi guy has given up and stupidly I havent chased baxi and 2 years has passed. I remember the corgi guy talking about the flame failure device not getting hot enough or being in the correct position but despite him adjusting the tip of the device to be in the flame as shown in the manual the fault persisted. I am wondering if this is a well known design fault or have I got a bad batch or something else wrong. Oh and just in case its relevant my Robinson Willey WARMPLAN balanced flue wall heater is fine too and on the same 15mm supply. Final thing: It doesnt matter whether I am trying to light a fire when no other house appliances are drawing gas or whether all the other fires are on so not the gas pressure?? Help! and Thanks for reading this