Front sights can be surprisingly tricky, even with "standards" like Meprolight. It COULD be that the sight retaining screw is too long, and "bottoms out" in the body of the sight without securing the sight fully. The instructions that come with just about every Glock sight set with a screw-on front warn that this can happen, and suggest that the installer do a test run with the sight tool and screw before applying threadlocker (to make sure the screw properly fits and secures the sight). If this happened to your sight, a replacement screw will be needed that fits properly.
Assuming that it's not merely a failure of the threadlocking compound, the installer could have also cross-threaded the sight screw. Probably not, but possible.
In order for Loc-Tite or similar threadlockers to set-up properly, the sight screw and sight body threads must (MUST!!!) be degreased. Otherwise, the Loc-Tite doesn't cure, and the screw will loosen under recoil. The Loc-Tite needs 24 hours to cure to full-strength. 2 hours minimum before shooting for it to "set", although I recommend 24hrs anyway before shooting. You can start a debate around here with the mere mention of the merits of "red" (high-strength) vs. "blue" (mid-strength) Loc-tite. I'm a "red" kinda-guy. Either will work provided the screw and threads are degreased, and the Loc-Tited parts are allowed to properly cure. Red works "betterer". It just requires divine intervention to separate the screw from the sight once it cures.
Take it back to the gunsmith and ask him to take a look at it. You could ask him if he degreased the screw and sight threads (many skip this PITA stage) prior to using Loc-Tite. If he says he didn't use Loc-Tite (or a similar threadlocker like Permatex), beat a strategic retreat to a competent gunsmith.
I hope this helps.