Ajax events sporadically working

i have three fields: a textbox (currency), a checkbox, and a selectfield (dropdown box).
I have set an ajax event to force the dropdown reload based on textbox {txtbx} as WHERE clause filter. (SELECT … WHERE money >= {txtbcx}
when i click the checkbox. i.e after entering the amount, i click on the checkbox, which triggers a reload on the select field based on the amount in the textbox.

This works sometimes, sometimes it doesnt. other times i have to do it over and over again to work.
i have tried substituting it with a radio button same effect