Couple of things you want to check:

1. If you manually activated the plug-in, cross check if there is no custom business logic inteferring with look up rules.

2. Verify following troubleshooting steps from wiki:

If the custom data lookup definition rules are not behaving as expected, check for the following conditions.

• Verify that the data lookup definition is set to run on the appropriate events.
• Verify that the matcher field is not read-only. Since users cannot change read-only fields, user interactions cannot trigger an on form change event for read-only fields.
• Verify a client script is not changing a field value. Client scripts can trigger Run on form change events even on read-only fields.
• Verify that the data in the matcher table is correct.
• If the lookup requires an exact match, verify that there is a matcher table row for each possible combination (including blank values). The lookup fails if cannot find a matching value.
• Verify that you have not created a recursive rule, such as:

  If Field A = 1, then Field B =2. If Field B = 2, then Field A = 2