I am on my third heater exchanger?

I am on my 3rd heat exchanger (37cdi Combi)
We had major loss of bar pressure on the 1st one just after installation. Worcester bosch (W/B) engineer came out tell us we must have a leak and to get our fitter to check my heating sysetm. For a whole day we had 2 lads looking under and over everything and everywhere, no leaks found. This cost us £120 for the labour.
Called out W/B engineer again did another check with plastic bags and turned certain pipes off. Still no joy so he ordered another Heat exchanger. Fitted the following week, and to be fair was'nt as bad as the first one. About 6 weeks later same proplem starting happening we lost bar pressure and was down on to the red. I topped up the system and again not to bad for awhile but still losing Bar pressure.
Called W/B again, out come the engineer. Gave the full story and he said i have a spare exchanger in the van will change it.
I have had all types of engineers out at all levels of rank.
This last exchanger has been working well but just lately the same loss of Bar pressure as stared again.