I offer tuition to children from my home. The parents are asked to pay me by bank transfer at the start of the month and this is written out in their contract. Some folks do this manually and others set up a standing order where the bank automatically sends my fee across. 

G has sent their children for lessons with me since May 2018. In October G was 2 weeks late paying me. In November G was 3 weeks late paying me. Both months I texted reminders and reminded G face to face. G never brought it up with me or premptively apologised for paying me late, it was me who had to bring it up with them each time. G also never mentioned a reason or apologised for paying me late just that "it's been paid now".

After being paid 3 weeks late in November I made it very clear that according to our contract I was to be paid at the start of the month. I asked that they do this in future and consider setting up a standing order. In December a few days passed without payment and I contacted G again. They said they were unsure about my holiday dates and told me that they planned to take Christmas day off that month and asked how much that changed their fee to (it actually didn't change their fee. My fee is averaged out each month and this is detailed in our contract). I confirmed that I would also be taking Christmas day off and asked them to pay me. G paid me and said they would set up a standing order for January.

This month 12 days passed with no payment. I did not chase G for my fee this time. Instead I gave them 10 days grace (taught their kids once in that time span) and then offered their lesson time to someone else. I texted them yesterday to say I didn't think things were working out and gave them a couple numbers for other tutors to try. G said they were "very sorry", they had set up a standing order but saw that it had accidentally been set up for 1st December 2019 instead of 1st January 2019. They sent me a screenshot of this. I replied to say that I had already passed their lesson time on to someone else. 

Am I the asshole for not chasing them up a 4th month in a row? If the standing order mistake is true then am I the asshole for assuming that they just forgot to pay me again? I saw them FTF last week and G didn't mention setting up a standing order and didn't ask if the fee had come through. Am I the asshole for not asking them?

Other points that made me decide to finally drop them: they only paid me on time in June and September 2018. They paid me late in May, October, November, December 2018 and not at all this month. They took 10 weeks off over summer which is unusually long (other students take 3-6 weeks off over summer) so no fees paid in July/August. The kids turned up 50% of the time without their books. It took G from May to late September 2018 to buy 2/3 books I requested they purchase and I spent each week prior working from old books or photocopies that I provided. G never bought the 3rd book I asked them to get.