A few things are worth trying before you give up on the date.

Check the last slots of the day. Late afternoon tend to hold availability longest, and both are better visits anyway.

Check again a day or two out. Cancelled and unclaimed slots sometimes do reappear, so a date that shows nothing three weeks ahead is not always closed for good. However, I will say from experience that these last-minute tickets are less likely to show up on the last day than some other museums and attractions.

Consider moving your museum day rather than your slot. Amsterdam's other major collections rarely sell out on the same days, so it is often easier to swap the Van Gogh Museum to a different morning than to squeeze into a full one.

Resellers like GetYourGuide have the same ticket avaialbility as the official shop, and if anything, I've actually seen fewer dates and times available on GetYourGuide than on the official website, so this is likely NOT a viable workaround (but you can of course try!)

If the date really will not work, the best answer is to change the day of your visit.