Serial Communication Textbox Refresh Weird Issue

I have this nagging issue that i cant seem to shake!
I'm reading in Serial from an HC-06 and Arduino. I take the Serial and print it in a text box for user feedback when they make a command via the APP.
I initialize the serial to a local variable when it comes in and print it into a textbox.
When the textbox gets full (if textbox length = x amount of characters), I set the text box to " " (nothing), then merge text box with the local variable (incoming serial).

So sometimes when I set the textbox to (" ") it will miss some of my serial command coming in. say i was printing the word "potato", so one item, not stringing serial together even - the first line of the text box will sometimes read "otato" or "tato", then the rest of the inputs will come in perfectly fine.
it only happens sometimes, and only if the textbox is cleared - what gives?

Thanks for any help with this!

It would be easier to help if you included a capture of your blocks workspace, but assuming there isn't a logic bug in your program, it may be the case that depending on the speed of the serial connection there might be data coming while the blocks are executing, so that data is lost. What baud are you using for the serial?