The mouth is a dynamic functioning system and jaw movements and tooth position need to be in harmony with each other for teeth and restorations to last. When you originally had your work carried out it was obviously done to a very high standard for it to last 20 years.
When the tooth postion and jaw movemnts are not in harmony with one another, some teeth will be subjected to more pressure than they should. Due to this the body tries to restablish the equilibrium. It can do this in a few ways.
1) if the gums are weak the pressure will cause the teeth to become moblie and physically move out of the way so there is no restriction to jaw movements.
2)If gums and bone support is good the body will respond by wearing away the teeth. This may have been your original problem.
3) if the teeth are restored, excess pressure will result in the failure of that restoration. This will probably be the reason for failure of your front two teeth.

When a crown breaks off, some of the tooth usually breaks off with it. The two fractured parts can be approximated easily, the old crown can used again. This will have been the case when you first had a a post fit. This time, it was probably not very easy to fit the two parts together, therefore the the fit of the crown was no longer accurate. Even if the post hole was drilled straight down the long axis of the tooth, The best fit of the crown may have been slightly further forward. There is a possibilty, of course, that the post hole was not down the long axis of the tooth.
Due to the fact that the crown is now out of position and probably has a thicker layer of cement around the margin and the post may not be in the right position it would be advisable to have a completely new restoration placed. 
Bearing in mind what I have mentioned previously, if the new crown is placed in the same postion as the old without looking at your chewing patterns it will also fail. There may need to be some adjustment of the lower teeth. Any crown placed on this tooth will not last as long as the other restorations that have been previously fit. The reason for this is everytime a new restoration is placed or a tooth is worked on it does deteriorate slightly and becomes weaker. The act of drilling causes micro fractures in the tooth. This is even more so in the root as the it is so much weaker than enamel.
I hope this answers your query.
Kind regards