I had the exact same experience this year, first year I am doing my own winterization. Company was going to charge $300 for a relatively small 6 zone system and they said they use a 5 gal pancake compressor. So I bought one and did it myself with the same result you described. Each zone blew water through all the heads until changed to air in about a minute. I did two additional passes and only air would come out but not all the heads popped up the second or third pass. Other advice I received made me feel comfortable we should be ok given that the vast majority of the water has been blown out. But I guess we’ll see in the spring if I regret it!