Last year I was 1 or 2 days into demo, had a dumpster in the driveway, and had not pulled permits yet- the inspector stopped and knocked on the door, was nice about it but gave me only the rest of the day to pull the permits without fines.  

Send another letter, call, knock on her door.

Rather than calling the manager to deal with it, you might find yourself saying knock it off directly to them which is understandable because the property and the neighbors are yours.2.

Should we just knock on the doors and ask the tenants themselves? 

Door knocking, mailers, advertising, word of mouth, etc, the same way an agent would find listings.

(knock on wood):) Since we had some success and learned a lot from my this property we thought we would start looking for our 2nd property in the same town and area.  

And you can learn RE through the school of hard knocks...major in something that brings you joy...irrespective of what anyone tells you! 

Phone calls....door knocking...and other door knocking.  

I once had tenants call me telling me a woman had knocked on their door telling them she had sent someone $750 (the rent was, of course, higher than this) and was upset to find tenants living there.  

As long as you have been operating within the statutes of the local landlord/tenant law and the terms of your contract (the lease), then it sounds like it is time to serve notice and begin the eviction process.However, you might try a knock on the door first and see if a calm tête-à-tête might resolve things.