Does that mean that files smaller than 300kB are stored correctly while the larger ones are not? Are you using the FILESTREAM provider? If so, could you send the output the following commands on the server?
sp_configure 'filestream access level'
select * from mssqlrbs.rbs_blob_stores
RBS also has its own logging that may point you to the cause of the problem. Depending on your configuration, you may find the logs in text files in the application directory or in the Windows Event Log.
To answer your second question, RBS installer doesn't uninstall the server side components by default. If you are using the graphical interface, you should get asked whether you want to remove them. Otherwise, you can use the following stored procedures:
[mssqlrbs_filestream].[rbs_fs_sp_uninstall] <schema_suffix>, <force_uninstall> - removes the FILESTREAM blob store. Schema suffix identifies the store and can be found in mssqlrbs.rbs_blob_stores, force uninstall controls whether the uninstallation
should continue even if there are files stored in the blob store.
[mssqlrbs].[rbs_sp_uninstall_rbs] <force_uninstall> - removes all RBS objects, force uninstall serves the same purpose as above.
The flegroup is not created by RBS setup, and has to be removed manually using the ALTER DATABASE command:
ALTER DATABASE <database name> REMOVE FILEGROUP <filegroup name>