villagers ive in my base have been despawning constantly

at first i thought they might've died to mobs so i lit up the area and added more iron golems and more light sources , then they continued to despawn , i have the essentials for the villagers such as a bell bed and profession block , i keep them in a small enclosed area so they cant escape , but they seem to keep despawning even though i have traded with them, there are more than enough beds for them , i started of with 15 then this number gradually decreased as time passed  now im left with 6 villagers