Probably because you are stimulating your cannabinoid receptors too often and with too much THC.
When you initially start to medicate, either recently or decades ago, your endocannabinoid receptors we smaller, less abundant and probably not as efficient. As you start introducing Cannabis they up regulate which means more are produced and they are more efficient. It’s like the body wants to be able to manage the intake of more Cannabinoids.
As you keep medicating, usually more often and usually with higher THC concentrations, these receptors become saturated and now instead of up regulating, they down regulate. Now it requires more and more THC to get the same effect you got before. This is called tolerance.
When this happens it might be time to do a T-break or tolerance break. Which might sound easier said than done since for most of us this has now become medicine, not just something we do on a Friday afternoon just to get high. Once these receptors stop being overloaded they will begin to up regulate again in just a few short days and then you can resume consumption.
To keep your tolerance low:
Use the least amonut of THC needed to get the desired effect. This should be your optimal dose. Taking more than your optimal dose will only lead to quicker tolerance and undesired side effects.
The oral route, tinctures, capsule, edibles, last longer leading to medicating less often and less tolerance. The inhalation route leads to a kicker tolerance since it has a shorter half life and requires more treatments during the day.
Avoid concentrates, obviously they will have more THC and saturate your receptors quicker.
Try to incorporate regular T-breaks into your regimen. They will save you money on the long run and you will get better relief when you do medicate.
Remember that THC is not the only Cannabinoid out there, CBD, CBG, CBN among others can also help you take a break from THC.