calculated Item Become Unsupported for a very long time

[BlackCrystal]

i have this calculated item which returns the tps.

(abschange("db.tps_acc[]"))/60

sometimes ( maybe twice a day ) the db.tps_acc become not supported which this calculated item will become not supported too but db.tps_acc is unsupported only for a short interval and it become fine
but this calculated item stay not supported for a long time like fpr 7 intervals.
why this happen and how i can fix this?