My advice: Unless you volunteer to give the home owner some of your finds ... there isn't any reason to permit you or anyone to dig around their property except to be "nice". What the homeowner wants is something in return to their giving you permission to hunt. It may be just acknowledging that you and they are like-minded "nice" people, so therefore you can hunt since you are "like" me. It may be that you are viewed "beneath" them, but they condescend to show mercy on you (which strokes their ego). It may be they are bored and they are excited by your enthusiasm and what might be found (lot's of bored people out there).
The superficial things under your control may be rather minor but they can stop you cold, it is your first hurdle. Tangible things are like: your appearance reflects a lack of attention, your voice inflection hints of deceit, your eyes reveal some hypocrisy as they dart about, your posture, your hand movements, even your smell .... the homeowner will size you up in the first 20 seconds: either disliking you (you're done) or liking you (maybe you can hunt ...they still need the intangible need being met though).
When your personal negatives are high they trump any other altruistic needs in the homeowner, and you're done. I'm not sure people are aware at all times just how offensive they can appear in the eyes of other people. Just chewing gum while you're talking tell volumes ... you aren't detecting today on my lawn. Do you smell like tobacco or beer ... bye bye. Are you wearing shades and a hat at the door ... so long buddy. Are you talking too close ... well then get lost. Do you look disheveled or unclean ... please go away. Are you talking unnaturally fast ... you're a liar, off with you. Why didn't you shave and why is your hair long and greasy - are you a rebel? ... well you're not touching my lawn rebel. What's with the shovel I see in your truck ... sorry, no!
There are of course homeowners who appear to lack graciousness and appear to have zero kindness. They detest everything and everyone and enjoy being rude and offensive. You'll get nothing from them unless you are very adept at sizing up the needs in other people (ornery people have feelings too, if you connect with them at some level the wall can be brought down, but it is much more difficult). For instance, a mean bitter-snake of a man became a different person when I agreed with his assessment of liberals and other political views ... that was the key to ceasing hostilities. With some people it's more of a puzzle ... they're angry but your job is to find the puzzle piece to reveal and unlock their bitterness - after that you are their "friend" (as strong as they hated you now they equally strongly like you - these people see things black or white, you just have to find a way to flip their opinion switch - it can be done).
So, everyone gets something in the transaction at their doorstep. You get to detect, of course, and they get to validate that they are a nice person, or that they are not unloving, or unkind, or whatever negative feelings they are trying to overcome at that moment (people want to be liked - the ego need is king in most cases). It could also be that the lonely homeowner gets to talk with someone - that's their payment. You have to give something to them before any deal can be consummated (it always happens whether you realize it or not).
When the homeowner receives a benefit from the interaction you get your permission, not before. Sizing up the situation so as to give the person that unspoken intangible "thing" that they seek is the art of the deal, it is what makes the sale. You have to be aware of what gets you accepted and be seen as acceptable in their eyes (not yours). Some people are more empathetic and personable (meaning they project that intangible "thing" better) so they're the ones that get to detect that old house with a nice yard, and you don't. They have something you don't - but you could if you worked at it.
The more you knock on doors the better you become at sizing up the homeowner and adapting your presentation to the unspoken needs of the homeowner. This works well whenever you deal with people - being able to size up the other person and say the "right" thing to win them to your side takes practice (you have to be keenly aware of their tells). But it will get you that refund at the store, get you out of a ticket at the courthouse, will get you the job during the interview, win over your in-laws, and so on.
That's my opinion on knocking on doors and how to be more successful (if you are already then you may just be a natural - some people learned how to read others at an early age and are very hard to say "no" to, they've perfected the art of the deal, so to speak).