Their new product launch was exciting, but it fizzled out after a few weeks.

fizzled 是动词 fizzle 的过去式。fizzle 原指"嘶嘶声"(像汽水冒泡的声音)，引申为"逐渐减弱/失败"的意思。fizzle out 这个短语表示"虎头蛇尾地结束"或"逐渐销声匿迹"。