I created content based retention policy. It should be triggered 2 months after Last modified date.
It is executed properly for new items. Items created before the policy was applied are not affected by the policy.
How to force it on all items of this content type?
There are two timer jobs that effect this. The Information Management Policy job is responsible for assigning the policy to the items and will set the effective date. The Expiration Policy job will then remove the items.
Check the compliance details for your older items and see if they have a scheduled occurrence date set. If not the IMP job needs to be run.
As you mentioned, your policy works on new items, that means both the timer jobs as described by Bunzab are running fine. The Retention policy works on all the items having that content type which the policy is set on. The only reason I think can cause this issue is because either the event criteria has not met or the item is not of the content type for which the policy is defined.