I have a mim fender squire. I'll be playing my guitar and over a short time my string tension loosens. I just put on some new (used) locking tuners and a better bridge. What else can I do to fix this problem?
If strings slip, even without locking attachments, first thing I'd look at are the machine-head windings.
If they're perfectly aligned, done carefully, they won't slip, ever.
If it looks like an explosion in a wire factory, your chances are commensurately lower.
Pic from Mike & Mike's Guitar Bar
The entire page at Mike & Mike's Guitar Bar shows you how to achieve this - though it really isn't hard to figure out for yourself how to do it properly. No-one taught me, I just arrived at it through common sense.
On a strat without tuning locks, then dipping the trem can make strings go sharp, as it allows the strings to slip at the nut.
My 'fix' for that has always been, you either tune strings by bending then checking, or you do it by dipping the trem then checking. You can't have both.
That way, if a string goes out as you're playing, you correct it depending on your initial method, either dip the trem or bend the string.
Personally, my choice to eliminate that entirely is to screw the trem hard to the body & never touch it again ;)
It is also possible your tuning pegs are don't bear the tension well. On my guitar the pegs have screws that allow you to adjust the peg tension. Mini-squires have cheap pegs that don't allow you to do this.
If nothing else works replace the tuning pegs although I would be inclined to just buy a better guitar.