Usually plugin changes are performed in a test environment. Ensure the new plugin jar files are located in the correct plugins directory.
By default Capital will read plugins from the local <Capital Home>\plugins directory. The background task could get executed on any of the CIS worker machines, so the updated custom task jar should be placed in each of the worker machines.
Alternatively, the plugins directory can be setup in a common place for all client applications via Capital Project Manager - System - Preferences - Paths: