In my early days as a reader, I would sometimes draw a blank. Now, I just push through any resistance. It's my job. I do this because I love Tarot, but also because I'm committed to working with 'spirit' and if they come to me, it's because they are meant to be there.
As someone said earlier, the 'bad vibe' is more likely to be me, than them. In fact, when I feel a reading starts off badly, with me not liking the person sitting opposite me, it almost always ends up with me feeling a great rush of love and empahty for the person when I push aside my judgements and just 'be' with them, where they are, in the moment.
The worst I have ever felt in a reading was one I did for a serial rapist and murderer. I will never forget it. The hatred in his eyes when he talked about women was chilling. He was recently out of jail (he didn't tell me his crime, but it was clear as day from the cards and my spirit friends who give me messages) and wanted to know how his arranged marriage would work out. He had converted to another religion whilst in jail.