its been a while since i did power, so i'm a little rusty:
KlausST is right there are several issues (several things to discuss) in your post.
if you follow the current path through the circuit for one thyrister on, you should find no path, and therefore no current flow
no current, no latching on
what you are seeing in the VP1 trace is the thyrister turning on
your simulator is giving you the voltage between VP1 and ground, not the voltage across the load
you must turn on two thyristers at a time to get conduction
when you have conduction, you should get latch on
suggestions:
change the three negative thyrister to diodes, or turn them on all the time
look at the voltage across the load (differential probes)
you should get what you expect
and do what KlausST said