Question regarding Zerodha Available Margin.

Hello Everyone. I'm new to investing. It's been 2-3 months that I'm using Zerodha app for buying stocks. I have one doubt, regarding Zerodha Available Margin. It might sound dumb but, I want to clear the doubt. I added some funds and is being reflected under the "Equity" Section on the "Available Margin (Cash + Collateral). So, I have created a gtt for a particular share. My doubt is that, will the added amount vanish after certain period of time. I'm asking this because, couple of months ago, as far as I can recall, I add added some funds and bought some shares. There was some remaining funds, like around ₹150-200 (I don't remember the exact amount, but it was between 150-200). When I checked after a month or so, I couldn't see the amount. I could only see ₹0 as my available margin. Has anyone experienced similar thing?