I don’t do that. That might be a shrader thing, but I think it just bumps it off the presta valve. You also might need to open the valve all the way until it won’t turn anymore.

Maybe try flipping it the other way? Idk I’m not an expert but for some reason my old pumps you flip the lever down 90° to lock it but on my newer pumps you flip it straight (so it’s pointing the same direction as the valve, towards the center of the wheel).

When using a new pump I just keep flipping it differently until it works, and I generally don’t have problems with presta. Not sure if some pumps make you flip it differently depending on the valve type - it seems like they all invent their own way to support both so it’s confusing.