Reed Switches - Good brand, stuck in the closed position problem
Reed switches are very simple devices and usually have only a few failure modes.  Here's some advice on how to troubleshoot:
The reeds themselves have to be magnetically very &quot;soft,&quot; and if you are purchasing them from a source that does not do a good job of material selection and annealing, the reeds themselves can become slightly magnetized by an external magnetic field and &quot;stick.&quot;  You can determine if this is your problem if your reed switches fail in this fashion when using a meter to measure the contact pins, (or if they are failing in a circuit where the current is low).  If this is a critical application, I would suggest purchasing through a reputable electronics distributor.
If your reeds don't stick with a meter and are only sticking in your device, it may be possible that your energizing magnet is still too close when you want to open.  When the magnet moves closer to the switch, there is a gap between the reeds causing a higher reluctance, so it takes a larger field to magnetize the two reeds until they are attracted together.  Once in contact, the reluctance is lower so the field strength to keep them magnetized is less than that required to pull them together.  This means that the magnet position at which the reed switch opens must be farther away from the reeds than the position when the switch pulls in.  Your problem could be this, and could be made worse if the required field strength varies from switch to switch.  Once again, I would recommend buying from a reputable supplier; good reed switch manufacturers usually specify a narrow range of field strengths.
Bending of leads can change a reed switch's performance if not done properly.  Leads must be supported during bending so that the bending stress is not experienced by the glass capsule.  I recommend the glass over the plastic because the contacts are in a hermetically sealed atmosphere for long life.
Finally, as @Jasen correctly points out, if you are switching any capacitive loads you can be introducing instantaneous currents capable of welding the contact surfaces.  This will happen regardless of the supplier in a poorly designed circuit.  For sensing applications, make sure you have series resistance to limit the instantaneous current through the reeds.
Good luck!