Just because you dont witness a lock doesnt mean they arent locking. Make sure you arent checking on them too often and disturbing them also. I have clear tubs so I can usually see a lock with a flashlight and dont have to move the tub. Remember, breeding usually takes place in the early morning hours and a lock may not last long enough for you to catch it. I would just keep pairing 4 days on 4 days off or whatever your preferred method is. I have a spider that locks for 2-3 days at a time and a pastel that only locks for an hour or 2. It just depends on their stamina and willingness to breed. Like someone else said just because they have the tools to breed doesnt mean they have the "know how" yet. Someone may own a hammer that doesnt mean they can build a house, ya know? Stick with it, they will get the hang of it