Mobile menu malfunction. shows only for a sec

So I have this site and the menu used to work fine on smartphones, but lately it seems to be malfunctioning. when you click on the dropdown mobile menu, it displays for like a second then disappears.

I am not sure even where to start in fixing it.

I tried to troubleshoot any javascript errors using google chrome developers emulator, set it to android s4, but the menu seems to work fine under their emulation, so there is no javascript errors. only on a real smartphone.

the site is https://teiinc.org. they are a non-profit and im just trying to help them get it working. Any help is appreciated.

owner is not sure when it started malfunctioning.

thanks