im having a problem with office thresholds, here is the setup, the main hallway is tile, the realstuff and its about 3/4 inch thick, the offices are carpet, never had thresholds before ,the ladys are tired of stepping on the carpet tack strip tacks as they run around sholess in there ,[i know just put yer shoes back on, but...] so i put in some low thresholds, they didnt want the doors trimed on the bottom and there is very little space under them, they work clearance wise but after a week or 2 the screws pull out, the "good" hardware store here has limited stock and the other one has very little stock, im using plastic inserts placed in holes i drilled into the office slab which is concrete, the way the door jamb, the carpet and the tile all come togeather leaves about a 3/4 inch valley between the tack strip and the tile right where the threshold has to be, its 3/4 on one side and about1/2 on th eother, ive used longer screws, and ive used bigger screws[ number 8's yesterday] but cant get them to stay, this is a lawyers office complex and the foot traffic is high any ideas? or a pic of anything that will hold these things in there, getting tired of doing the same job over and for free