First, if the strobe has an adjustment for speed of flash, check that.

Second, check that the battery is charged. When they're low, strobes slow down.

Third, re-check all your connections. If the solder joints aren't shiny-silvery and smooth looking, get your soldering gun and some flux out and re-make the joint. That may even include cutting the wires off, restripping them, and doing the connection all over again. (Besides, that's good practice at soldering! )