Bootcamp Assistant and 'lost' disk space

Bootcamp Assistant has known issues during the partitioning phase which can leave the disk space on your internal disk in an unusable state. Apple's use of CoreStorage as the current default (and APFS in macOS 10.13), can also complicate recovery. If you use FileVault encryption, your macOS volume will also use CoreStorage.

1. If you have Free Space in the CoreStorage volume, it is easier to rectify. diskutil cs resizeVolume and diskutil repairDisk can be used. See Bootcamp drive/partition not showing up in Windows 10 setup screen. as an example.
2. If you have Free Space in the underlying partition, GPT fdisk download | SourceForge.net can be used to create a 'fake' partition, which can later be removed using Bootcamp Assistant. See Can't get rid of a windows partition? Help? as an example.
3. If you have a Fusion drive, see Lost space after removing Boot Camp partition for an example of creating a GPT entry to remove the Bootcamp partition using BC Assistant.
4. If you have an APFS file system on High Sierra, also see Bootcamp "The startup disk cannot be partitioned or restored to a single partition" error. as an example of various lingering partitions and a possible solution using diskutil commands.

If your issue cannot be solved using either of these suggested techniques, post a new discussion in Boot Camp and the ASC can assist in helping resolve your issue.