A basic problem is that the British side is run by politicians and the European side by bureaucrats. As a sweeping generlisation, politicians do not mean what they say, are willing to vote tactically and to change allegiance when it suits whereas bureaucrats work to rules that, once established, can only be changed with the greatest of difficulty.
The British side interprets what the European side says as if it had been said by politicians and the European side interprets what the British side says as if it had been said by bureaucrats. This leads the British side to wrongly assume that European positions taken in the negotiation are simply tactical manoeuvres that can be negotiated away later. It leads the European side to see the British as incompetent, shilly-shallying and not knowing their business.
The British think that what matter is what happens in the UK Parliament and there what matters is party politics, leaders hanging onto power even when they do not really represent their own followers and issues of party loyalty. All this, however, is a side show that contributes virtually nothing useful to the Brexit process, merely to posturing by figures who then appear in the newspapers. Thus the popular press this morning portray last night in Parliament as a great triumph for the Prime Minister Theresa May, even though what she achieved was to get a parliamentary majority for the rejection of the position she herself had negotiated and three days ago claimed to be immutable. This majority in parliament obliges her to go back to Brussels to renegotiate the Irish part of the exit deal. Within ten minutes, the EU side had already said, this is not possible and there is no point in meeting. Ah well.
It would all be very amusing if it did not have such serious consequences, especially for Britain, but in varying degrees for many other countries as well. At a time when the wholer world economy is on something of a cliff edge, this is really the last thing we need.