You say your trend line disappears when you switch from a 1-hour chart to a 15-minute chart on MT4. Have you taken into account the fact that your Y-axis most likely displays a smaller range of prices when you switch to a shorter time frame chart?
Please let me know if you can find the missing trend line by adjusting the Y-axis on your 15-minute chart. (Hover your mouse over the the prices at the first right of your chart, so that a double-sided arrow pointing both up and down appears next to your mouse icon. Then, click and hold your mouse button as you drag your mouse downward to increase the range of prices displayed on your chart.)