I did have the same exact issue a couple of days back when i tried to create a deposit address in Poloniex,it says that you need to have 20 XRP for creating a deposit address and i got the wallet address but when i went to bittrex ,i tried to enter the wallet address and an addition space for tag is there but i cannot see any tag value in poloniex ,i am not sure what to add there ,so i left that empty and the coins never went through which was obvious,i am sure that i was able to withdraw a huge number of ripple a couple of weeks back from bittrex,but now i am really confused,even i need to have some clarity regarding this,did you contact poloniex support team regarding this,as i think they have to provide the tag value.
There is a bug in Poloniex wallet and also in Bittrex wallet. I (partially) solved it by sending 50 coins form Kraken to Poloniex.. that was accepted and then the wallet was funded.
The last problem is that those coins did not show up in my Polo wallet for a few hours. A few hours later the coins arrived.
Now Bittrex allow me to send the coins to polo.
The unresolved issue is that I cannot see the proper balance of coins in my XRP wallet at polo via XRP explorer. I guess that is not so important.. but this is all strange.
I asked on Polo Trollbox and they said that polo does not give you atag value but a wallet address of your own so you do not need to put anything in the tag value.