This usually occurs when your Gel Nail Polish is stored incorrectly, the rim/neck was not wiped after use or it was used too close to natural or artificial UV rays.To fix this, give the brush a gentle clean with rubbing alcohol or isopropyl alcohol. Make sure to throughly rinse the brush with warm water, then dry with a Lint Free Wipe before putting it back in the bottle. Do not use anything acetone based to clean the brush, because this can melt the brush. To avoid this in future, please make sure to follow the storage instructions outlined on the product page.