I have been using a pendulum successfully both personally and professionally for over 25 years, and have made some observations in the process.
What I have discovered is that the pendulum is only as good as you the user are impartial and detatched from the result, and that's not as easy as you might think. Another person who has no expectations may be better to dowse on your behalf, because you would be suprised to find how deep your expectations went - right into the depths of your subconscious that will form the result.
I mainly use it to dowse substances, foods, crystals, remedies, etc to see if they would be good or bad for my own and others' health so in this sense I use as an alternative to muscle testing, and it's been 100% accurate in this respect. Because we are all connected, and to all things in this universe, we can tune into any other person anywhere in the world and deduce very accurate information about their health for example, without knowing anything at all about them, and in some cases, not even their full name. But you have to be detached and have no expectations.
IMO, things it will never work for are any future events, normally. This is because our timelines are still in a flux up until the actual manifestation itself. There are too many variables, and your pendulum cannot possibly pick the most probable timeline, because even this will be subject to unforseen changes from the collective at any time.
It works best for physical and health issues, I feel, and deals best with facts more than conjecture.
Any issue where you have any emotional response to the question asked or the potential outcome, or an opinion or presumption, will affect the outcome, and you're likely to get the same result over and over again, which doesn't comfirm that it's working, just that your subconscious is driving it in the same way every time.
Also, you can inadvertently programme your responses in this way and the pendulum to react accordingly through repetition. This is especially true if you use a quartz crystal pendulum of some kind, as this will doubly reflect your own thoughts or subconscious thinking processes/habits, so try to be aware of this. You want your crystal to remain impartial as well as you.
Things to avoid are emotive issues such as
"does x love me?"
"Will I have any/another child?"
"When will I die?"
"was I a woman/man/ soldier etc in my last life?"
"Am I winning the lottery this year?"
Conjecture, past or future questions are no- no's, and so are relationship/love issues, because we normally have opinions, desires and or feelings, however deep, about these, so are highly unlikely to receive an accurate answer....What really is 'love', for example? Perhaps someone who hates you also loves you deep down and vice versa because some would say this is our true reality, but if you interpret that the wrong way you might make a plonker of yourself or upset someone else..! If you do go there, take it with more than a pinch of salt, then leave these tenses, potentialities, or complex issues, where they are.
Present tense, physical stuff, and facts are best, because the pendulum is a physical object resonating with your own etheric/physical energy field in this dimension, which is the present, the sum of all worlds. There may be millions of parallel or alternate universes where we also exist and things are slightly different, but this is the one you're physically inhabiting right now, hence, the information that you can reliably ascertain must resonate fully with this.
Trust your instincts fully and feel confident in your ability, but also try to be self aware, too and recognise any other impulses or feelings that could be inpeding your natural flow. Just observe. You will know when you've got the hang of it, and you'll feel sure of your ability. Namaste.