Inventory policies are not designed to run as soon as a policy is received by a newly managed machine. Typically the policy will wait until the scheduled time to run, which depending upon environment could be a week or more from when the newly managed machine first checks into the Notification Server.
There however is a method that can allow machines to report inventory within a few hours of joining the Notification Server. This method will require a new inventory policy, and a target created by two filters.
These two rules in conjunction will bring up any computer that has an inventory agent installed, but has not reported in its inventory. Upon reporting inventory, the next time the resource membership runs, and updates the filters and target, it will remove the computer from the policy.
Please note: Running this method of inventory collection can cause performance issues, especially if a large amount of new computers are joining the Notification Server. This method is intended for an environment where a handful of computers may be being brought in at a time, and for reporting purposes, inventory is needed as soon as possible.