Reason: This is primarily a problem with fragrance oils. Some don't do well when combined with raw soap and will bind with bits of hard fats in the recipe causing those solid lumps.
Solution: Carefully read reviews before purchasing and using a fragrance oil. Try using your immersion blender to smooth the lumps out. This will result in a very thick trace, but it might help save the batch. If you can't blend the majority of them out, then I suggest rebatching the soap.
If an essential oil causes ricing check the brand that you're using since it's likely cut with synthetics. Inexpensive essential oils from Amazon and Ebay, or local stores...are almost always mixed with synthetic fragrance to save cost. Pure essential oil should not cause ricing in soap.