I had set up a payment plan months ago, IRS still hasn’t withdrawn first payment. I was told to pay manually online, or should I wait till next month?

So I owe about 2000 to Irs. I had set up a payment since the beginning of March and to have it withdrawn on the 20th of each month. I called when it wasn’t withdrawn in March but I was really confused. I was told that it may take months for the first payment to be withdrawn and that I should just start paying it online. But the balance that I owe is not even up yet and will take time, so where would my money go? And if they don't take it on the 20th , will they just take it on any date they please, even after I set it up? Because I need the extra cash and I'll just wait till the next month to pay it either online manually or hopefully they'll withdraw it by then. Please help!