Could be, or could just be a bit cold for them. I would suggest asking them, but that wouldn’t be very helpful…
That should be 2:1 (sugar to water) or 5:3 if you want something easier to dissolve. 1:1 is spring or summer feed. The bees have to use energy to dehydrate the syrup for longer term storage. If you give them 1:1, that is a lot of work. Most nectar is closer to 2:1, so if you give them more concentrated syrup, it helps them out.
I do 2 other things with my syrup feeds, which are not commonly used by most beekeepers, but I have my reasons. Firstly, while I am boiling the water, I add a couple of “petals” from a star anise spice pod. This gives the feed a scent which bees seem to equate with food. The other thing I do is add a scant half teaspoon of Ascorbic Acid powder (Vitamin C) to the syrup once it is cool. This acidifies the syrup making it closer to nectar, and it also beautifully inhibits mold growth.
I only replace syrup if it is getting moldy, but that only happens if they aren’t taking it. In that case, you need to ask yourself why they aren’t taking it. Assuming that you have scented and acidified a nice concentrated syrup, the remaining questions are - is there a nectar flow, or is it too cold?