There are 3 main reasons why Deep Sky Stacker struggles to stack stars:
1 – The star detection threshold is too low (change setting 1, and read this FAQ about dim images and 16 bit stretching with Altair Cameras).
2 – The software is picking up noise or bright pixels instead of stars (try changing setting 2)
3 – The stars are too blurry (example 3 – improve your focusing with Bahtinov mask or using SharpCap PRO focus assist)
4 – A combination of all the above!