My initial gut instinct is that you have air getting in when trying to use it. It may not leak out, but once you open it up and apply all that suction power, the smallest of air intakes will be your demise. Perhaps the first time you got it going it was fine, and developed a leak from use. Could the pipe have collapsed from the pressure the first time, when you did not see it stop, and that has caused a breach? Possibly just moving the pipe around a few times has broken a very small area of a joint open? Fill cap is sealed tightly?

There are many members here that are very good with siphons. Hopefully they will have some ideas.

How about trying it with a regular hose, just to see if you can get that going? A lot easier to work with for testing. Seems like if you could get that going, a bigger version would work.