Other thoughts... if your level stays good for 1.5 hours, that's good. But 'good' could vary by 3-5 parts-per-million (ppm). You want a 4-7 ppm chlorine level for normal swimming, so you can see how a 3-5 ppm shift could be a big deal.
Also, the chlorine tablets will add chlorine and CYA. As was mentioned earlier, more CYA makes you need more chlorine. Most here will recommend adding just chlorine, and the cheapest chlorine is household bleach. (Yup, really.)
If you have 'ok' chlorine and CYA levels, you're usually fine. But to have 'ok' CYA and 'low' chlorine tells us your CYA level and chlorine level are out of sync. Adding tablets, or shock that has CYA in it, will raise your chlorine up... but also your CYA. (Sounds a little confusing, but you'll get it.)