I presume you mean the breathers on the heads.
This can be due to a couple of reasons: The breather `Umbrella` valves may have got dry & stiff, not allowing them to seal.
Oil may be pooling in the rockers then blowing through the breather.
You can space the umbrella valves a little higher which will stop the problem. There are a couple of kits available to do this.
This may help;
MKL's Harley-Davidson Sportster 1200 Custom - How to Deal With The Oil Blowby Phenomenon