What sort of block plane are you using? There are low angle planes, with the blade bedded at 12 deg. A 25deg bevel on the blade will give a cutting angle of 37 deg. This is good for planing end grain, or very straight grain, but will give you lots of tear out, if the grain changes.
Standard angle block planes are bedded at 20 deg. A 25 degree blade bevel will give a cutting angle of 45degrees, which will start to reduce tear out a little, but usually still too low for most tough bow wood. To get decent results you really need to sharpen the blade at least 35 deg to get 55deg cutting angle.
An alternative is to get a small wooden chinese or Hong kong style high angle mini smoothing plane(internet woodworking store eg Lee valley or japanwoodworker). They are relatively cheap, but perform well once you get used to them. Its a conventional plane, bevel down(opposite of a block plane), they are bedded at 55-60 degrees, already. Their blades are high speed steel, hard to sharpen but stay sharp for several bows. Also if your wood is still too tough you can flip the blade over like a block plane and it becomes a scraper plane, bedded at 85-90deg.
Always keep the blade really sharp.
Hamish.
