Alert Management was unstable

Dear Forums,

i try to create my own email alert management that trigger by Date and Time (eg. every Monday at 1.00 pm). But it doesn't work.. I try to create a new one with the same query but i set up every 1 hours...and i received twice only afterward no email again. For system alert management that trigger by document such journal entry are more accurate.. Does any on having the same problem? and how to overcome it. Thanks