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It's very hard to determine what's going on with the plant. The yellow leaves indicate some nutrient deficiency which could come from lack of nutrients in the soil, but also from other stuff like overwatering/ waterloggin or pest pressure.
I'd try to give it a little bit more nitrogen, but very carefully as not to burn it.
Fish fertilizer every two-three weeks is a godsend for me in poor soil. The risk of nitrogen burning is so low with this fertilizer.
It stinks a bit when applied, but only for a few hours.
Yeah those yellow leaves scream nitrogen deficiency - it always hits the older leaves first becuase the plant moves nitrogen from old to new growth when it's lacking.
Seems fine? Maybe just a little shy. A nitrogen hit won't hurt since these guys are mostly leaves and water, but maybe it's been a bit cool? Usually curcurbita don't really take off till the high heat comes in.
What's the weather been like?
Just finished a heat wave. And the other type of cucumber has taken off, so the weather seems good.
My diagnosis, then, is that it's shy. Try not to look directly at it. It's probably nervous from all the attention.
if you piss standing, it's easy to fix. Just "visit" that spot twice a week.