Gear change issue

[The Fridge]

Hi, wonder if anyone can help. Fairly new to road cycling and the mechanical issues that go along with it. My gear set up is compact 50/34 and 11-34 in back, gears change perfectly apart from changing into one gear only ie; 34 - 30 ( 2nd biggest cog in rear cassette ). Strange thing it goes into same cog when i'm in large chainring ( 50 ). So just now i'm riding without the use of the 34 - 30 gear ratio. Totally bamboozled .......

 

[The Fridge]

Yes, for some reason it completely jumps the 30 cog , going up and down. No problem tho on large chainring .........

 

[Ajax Bay]

Right.
The first thing I'd do is to carefully check the rear mech alignment ( @screenman). Make sure it is perfectly vertical, in both planes, particularly the plane perpendicular to the bike's plane.
Next @cyclops will share his preferred rear derailleur indexing video.
When the chain is in the smallest sprocket is the guide jockey wheel lined up?
Now click to the third largest 27t(ish) sprocket (chain on inner chainring): is it still perfectly lined up? (OP symptom suggests it will be off towards the spokes.)

 

[The Fridge]

Hi , thanks for your help. A friend had a look at it and sorted it in 2 minutes , all he did was fine tuned cable tension using adjuster till 30 cog was perfect , low and behold every other gear change was good also. I had set up gears initially referencing a GCN Youtube video, which seemed to workout ok, apart from one gear out of 20. Have came to the conclusion that gear setup / adjustment on a road bike is a black art !