Loop Breaks At Different Points

I have a sheet named SymbolList in which I list stock symbols in the first column, and have row headings in the first row. I use the following sub to get stock quotes from yahoo. I developed a loop that works in steps of 200 because yahoo has a limit of 200 stock symbols that can be acquired in each request. The loop should go through the whole list in loops of 200 until the last row or if by mistake there is an empty row. The problem I face is that the loop does not work in a consistent manner, for example if I use 200 as the step limit, the sub skips all loops and only gets the last one, if I use a limit of 100 the sub skips the first loop and then proceeds, it only works well at a limit of 50.