Strictly speaking, the ratio of (your culture / their culture) in each planet's hex must be >= 4.0(? -- if that's still the case).  If one of those planets is Iridia with culture of 180, then you need to project culture of 720.0+ into that hex (?? -- for k>1 turns?) to flip it.  Since you say all space hexes adjacent to the planet are in your ZOC (not the AI's), we can infer that your culture in those hexes > Iridium's culture.  But that could mean you out-culture Iridium (for example) 600 to 170, which wins the ZOC duel but not the flip.  Waza-ari, not ippon.

Along these lines, I'd pay a forum rep for a Culture 101 thread that shows us the math:

• Relation between pop and culture?  e.g. is base culture "produced" by pop, or only by special tiles?
• Most Culture Improvements give +% to cultural growth, not to base culture.  But the Planet cornerbar in the lower left corner shows your planet's Culture, not +cultural growth%.  Civilization Capital and wonders seem to give +base culture.  This is all very foggy -- hence a 101!
• Cultural growth per turn?  limit (if any)?
• Cultural decay function based on distance?
• Policy for combining multiple friendly sources of culture in one hex?  (N.B. this seems to be max, not add)
• Do starbase culture modules multiply:
• culture on all friendly planets in effect?  ==> you put them near your own planets
• "culture waveform" on all space hexes in effect? ==> still dunno if you put them near enemy planets, or near your own -- it depends on the shape of the decay function!
• Workflow for culture flip:
• Policy for threshold?  (Conjecture: simple ratio of 4.0)
• Policy for timing?  (Immediately?  or is there a delay?)

From 0.42 early-game plotting, I have some data on the cultural decay function.  It appears to be deterministic (whew!):

• (38.8, 46.8) = 09 hexes
• (47.0, 54.9) = 10 hexes
• (56.4, 66.3) = 11 hexes
• (66.7, 73.5) = 12 hexes

The intervals are all open, reflecting incomplete data between the endpoints.

• [UI-request] Space hexes also show all known culture scores.
• [UI-whimsy] Add a culture histogram UI overlay mode, where the entire galactic map shows an overlay with all known empires' cultural scores in every hex.  Then your 1-hex non-ZOC where some AI still beats you would appear naturally as an island (AI's culturegram) protruding above the waterline (your culturegram).  "Culture duels", e.g. where your culture ratio is >2.0x (which suggests that you're going for the 4.0x flip), could be shown as a vertical progress bar, e.g. above each of your mountain peaks it shows a hollow cylinder, whose top is at the flip-threshold (4.0x), and it shows a fuel-tank mark from E(nemy) to F(lipped).  Then you could sweep your gaze over the galaxy and see who owns what, by how much, and how close you are to flipping those last holdouts.
• I would play a culture-flip game just to stare at this map and meditate ommmmm
  

Well I'm up to 200 turns now with the enemy planets within my borders. My cultural borders now fully consume the galaxy. I've surrounded the enemy planets with 4 fully upgraded culture starbases, and loaded my nearby planets (within 4 hexes) with extra influence improvements. Still no flipping. Wish I could see the influence ratio on those enemy planets so I can tell if I'm even close.

I had a similar issue in a game, I hope there is some indication of your progress toward flipping a world eventually.

But not as challenging.

I can understand that. Don't know that I'd have the patience though.

Save game, invade, look at his buildings/improvements , see what his base influence growth actually is see if there is anything that would prevent a culture flip, the reload

Invading doesn't reset current influence?  mu-ha-ha