You are not being unreasonable - it is natural to feel disappointed and a little hurt at the sense of being overlooked. But, I think this is a common thing, most especially with sons it seems. They all seem to live such busy and pressured lives these days.
My boys tend not to get a present/card to me on time. Sometimes, something arrives the next time I see them which could be a week - ten days after the date. Sometimes nothing at all. This year, I didn't see any of my 3 (DD1, DS1 and DS2) on Mothering Sunday and nothing from anyone - not a card or flower in sight - though three 'apologies'! To be honest, I felt I had to quietly 'deal' with this, compounded by the knowledge that they all gathered at their dad's for Father's Day later in the year - there had been very practical reasons why we didn't see each other around Mothering Sunday, but sigh! It was also my DD's second ever MS as a mum and I think she was very busy with work and probably very distracted by actually qualifying for a gift / card herself!
BUT, none of my children are 'thoughtless.' These things do just happen and I think we have to let the niggling disappointment/ hurt go as I am sure they do not intend to cause any upset.