If you are using the SMS Add-on for AMT in "Integrated with SCS" mode and you perform a client discovery using SCS, the Add-on records the time/date when it performed the discovery.
If you perform another discovery using SCS at some future date, the Add-on will only discover systems that have been provisioned since the last time the SCS discovery was run. As indicated by the first reply to your post, this was a design decision to optimize the discovery process by not re-detecting existing systems and only detecting systems that were provisioned since discovery was last run
In the case you mentioned, since you deleted the client resource from SMS and re-ran the SCS discovery it will not be re-detected by the Add-on because the client was not provisioned since last time discovery was run
If you want to avoid using an IP scan you can reset the last discovery time / date - there are some entries in the registry that are fairly intuitive to change although as always, you should exercise caution editing the registry