Did the mechanic/shop advise you to
bed the pads in gradually over a reasonable time/mileage? ie. by using progressively more brake, for longer, starting off gently braking and letting brakes have chance to cool between applications? If not pads may have glazed a little, it might be possible to roughen up the braking surface of the discs with abrasive paper/cloth, sandpaper/aluminium oxide paper/wet and dry/emery etc, and also the same to the braking surface of the pads if need be.
As mentioned above also, it is usual to apply a thin smear of copperslip or brake assembly paste to the back of the pads, any anti rattle shims and any sliding or retaining pins, BUT this must be kept clear of the braking surfaces!
If pads are not free to move easily in the caliper this can cause them to stick or drag, causing squeaking or squeeling, it is also possible on some bikes, depending on weather conditions of use to have the brake operating pistons start to stick, again stopping pads moving so easily away from disc when not braking.