Some possibilities:
You may have too many vines in each pot. You'll have better luck if you plant just one vine to a pot so that the roots have room to spread and no competition for water and nutrients.

Are you watering and fertilizing regularly? Veggies in pots need alot more attention to both in order to thrive. It is recommended that you put slow release fertilizer in the pot at planting time AND feed with liquid fertilizer at least twice a month too.

How old are these plants? Cucumbers and squash do not produce well forever. They produce a nice crop and then the plants begin to noticeably "age" (yours look like that) and they stop producing very much until they give it up. This year, I planted a second crop of cucs a month after the first ones. It worked out really well. My first crop is starting to play out and the second is beginning to bloom.

Looks like it may be beetles. Do the leaves get lacey and crumbly?

I'm thinking small pot with too much water. How am I doin?