Unfortunately, the hardware requirements for Cardboard still lean to higher-end devices than the majority of people own. I’ve tried deploying cardboard apps to a mishmash of devices but outside of iPhones and Samsung Galaxy S/Note series phones the results were very poor. Obviously flagship devices from HTC/LG and other well known manufacturers should work as well, including older flagships. But low end phones like the Galaxy A (except for the newest 2018 A8 models, which I believe will have Gear VR support) are a no-go, because their gyros tend not to be well calibrated or are missing important functionality.
It could be the headset you are using, if it is similar to the Homido mini and requires you to hold the phone with your fingers. Your hands shake quite a bit, something you don’t notice until you put magnifying glasses up to your eyes. Try a headset with a headstrap or one you can press against your face.
Finally, make sure you calibrate the Google VR app with the headset’s QR code (available online if you lost it). If you don’t, you may get double vision and will certainly experience more eyestrain than you would if it was set up correctly.