The word Clairvoyance evokes thoughts like "mind reading" and "seeing the future". It gets complicated with Hollywood's dramatic projections of what a Clairvoyant does.
Not to mention, we fear what we do not know. Fear spins tall tales.

The result is lot of distortion around this subject.
From a spiritual seeker's perspective, my notion of Clairvoyance is...."clear seeing".

Seeing or accessing information you need- insight into a situation or information to make a decision.

All of us are "wired" for Clairvoyance. Some of us access it consistently and some of us don't.

Try this.
Sit somewhere you are comfortable.
Close your eyes. Take couple of deep breaths.
With your eyes closed, see a picture of a red corvette, about few feet in front of you.

See it?
Good.

There, you just used your "wiring" or ability for Clairvoyance.

Your ability to “see” images and “read” (interpret information from the image) is your ability for Clairvoyance.
Spirit communicates in 'mental image pictures'.
Your ability to see “mental images” is also activating your “spiritual nature”.

The process is same, if you want insight into any situation.
Obviously there is more to this.

What separates a Clairvoyant who can “see” without doubt and a Clairvoyant who is starting out?

Besides confidence from lot of practice...ease and clarity of seeing.

But what influences these ?

The degree of "stuff", between you and what you are looking at,
influences how easy it's to see and how clearly you see.

I'm going to qualify "stuff" as…self-doubts, pain, not wanting to see, limiting thoughts and beliefs (yours and others), social programming etc.

Developing clairvoyance then becomes about…clearing the "stuff" in the way. Clearing those cobwebs around your natural born gift.

Even for experienced clairvoyants some topics are easier than others.
What they have difficulty looking at, has a charge(or maybe even a trigger) for them personally, hence influencing ease and clarity of seeing.

The degree of your clairvoyance then equates to.…how clear your "clairvoyant space" is.