I reckon you are not making the bom correct - compressing enough etc.
I also battled in the beginning with my boms. The shape and size of of the bom is also very important. Make it in an egg shape and not round. Make sure the bom rests on the sinker with the sinker at the bottom of the bom and centred. I am not sure on what size sinker you are using but for my long throws I normally use a P3 (M3) or P4 (M4) with a small bom. By using a small sinker like an M0 with a big bom for a long cast you may throw the bom off. Some people use roof washer on the M0 to get extra surface area for support of the bom.
I reckon 200m quite possible but 250m - that i gotta see

- I have fished next to a few Springbok anglers and fished with some of SA's top casters. Maybe 200-220m with a sinker and no bom.
By using a spring you will be able to use a bom mix which is either too wet or dry.
It will also help if you are not compressing the bom right - or if you not shaping right.
You can add a little cake flour to the bom - it will act as a binding agent - beware because this takes longer for the bon to break up in the water - it must break up as fast as possible in the water.
For my long throws I sometimes use a spring - i mainly use a spring for tempo fishing.