Why can i not charge my Weihrauch HW100S PCP Rifle to 200 bar?

i purchased the rifle today 2nd hand with diving bottle but i cannot charge the on-board air cylinder to the 200 bar max, its gets as far as 175 bar and will not go any further, the reading on the diving bottle says 180 bar when im doing this, do i need to get the diving bottle filled up?? im new to PCP rifles so its probably something basic that im messing up on, dont get me wrong, this is a really great PCP air rifle, and the accuracy is superb, i just dont really know what im doing with the pressurised air........any advice would be great.....? its the newer versiot with the quick fill, and the diving bottle is 300 bar NOT a 232......thanks!