There is one rule when it comes to blackmail: if you give in there is a good chance you will face more of it.
When Erdogan suddenly opened the borders and flooded Europe with "refugees" there was little doubt that this was blackmail. Yet soon lots of high level EU diplomats could be found in Turkey and when Erdogan a little later visited the EU he got a red carpet treatment and almost everything he had asked for. It will be made easier for Turks to get a visa for the EU - although the details still need to be negotiated -, Turkey will get 3 billion euro to support its refugees and an EU progress report that critical of the situation in Turkey was postponed until after the Turkish elections of 1 november.
To top it off Merkel visited Istanbul shortly afterwards and declared there that the agreement between the EU and Turkey on the refugees had brought a Turkish EU membership closer. She could as well have shouted "Vote for Erdogan!". Unsurprisingly Erdogan won the elections - getting much more votes than the opinion polls had predicted.
One can only wonder what made the Eurocrats so stupid that they gave in to Erdogan's blackmail - and doing so helped him win an election. For some of them it will have been mere stupidity. But I suspect that US pressure played an important role. A weakening of Erdogan's position would have put in doubt Turkey's commitment to America's regime change project in Syria. Obama has named Erdogan as one of his top five international friends. And it was another of Obama's top 5 - Merkel - who helped to explode the opening of Turkey's borders into a crisis by declaring begin september that Germany would accept any refugee from Syria.
Soon after the gates opened in Turkey articles started to appear that suggested that the flood of refugees was no coincidence but the result of deliberate decisions at the top in Turkey. This was confirmed by an Erdogan advisor.
It didn't help that the EU had egg on its face as its memberstates had been very busy slashing their contributions to the World Food Program, that as a consequence was forced to slash its support for Syrian refugees in Lebanon, Jordan and Turkey.
As could be expected Turkey has only partially stopped the flow of refugees since the agreement. It knows very well that once the pressure subsides the EU may start to come back on some of its promises. So it does enough to give the impression of an effort while it does on the other side little enough to keep up the pressure on the EU. And once the EU starts delivering it will without doubt find ways to increase its demands: asking for more money and also repeating its demands for a military intervention in Syria - its main demand that wasn't concurred with. According to Amnesty Turkey has found yet another method to keep up the pressure: maltreatment of refugees and even sending some back to Syria.
Recently we have seen the EU heaping the blame on Greece for not guarding its border better. But Greece doesn't have much incentive to do so as registering the refugees would also bring the obligation to keep them. And what sense does it make to register refugees when you can't send them back. Besides that: Greece registering the refugees would do nothing to stop the flow. It looks like Greece is singled out as a scapegoat by Eurocrats who don't want to anger Turkey.
The EU could have taken the opposite approach. It could have scolded Erdogan for unleashing the refugee flood. It could have declared very publicly that as long as Erdogan is in power an EU membership for Turkey is impossible. Punishment is sometimes a better strategy than rewarding blackmail.
Now the EU has adopted crazy plans to adopt 160.000 refugees. It will help nobody. There should be adequate money for refugee shelter and support in the region and also in Syria itself. The stated logic is that it is better when we make our own selection instead of waiting for who comes by himself. But such a selection will do nothing to stop the flow crossing illegally the borders. On the contrary: one can expect it to increase.