[Trillium]

Greens like pak choi and spinach can grow only in very cool weather. Add a bit of heat and they're finished. They also need regular rotation, even in the same year so you shouldn't be replanting in the same spot at any time.

[sunshineband]

Do have a look in the ground anyway, as it may be that you have either wireworm or cutworm (which are fat caterpillars really) and they can both eat through roots causing the plant to wither. 

They  can decimate rows of seedlings.

Pick them out by hand if you find any