It was the fault of post-commies, those ill prepared fools haven't been able to get it right. Still is was better than nothing.
Mind you, it would be wrong to lay all the blame on a single group of actors. If there was anything all the political actors in Poland could agree on back then, it was the demand of the final recognition of the Western Polish border (which was never in question in Germany, but Kohl insisted that such a binding legal agreement could only be made after German reunification, not before).
As there is no this alleged agreement from 1953 you claimed settled the issue. The only info about that mythical agreement from 1953 is a one article in a newspaper (a German newspaper).
I thought PiS called the agreement of 1953 invalid, thus admitting that such an agreement indeed happened. The treaty itself received little attention until PiS brought it up, because the Warsaw treaty in 1970 and the treaties of 1990 and 1991 are deemed more important overall and have settled the issue conclusively.
The some with those Kohl-Mazowiecki talks. It just smokescreen. Sure they were talking but nit about reparations.
The issue of reparations (and minority rights for Germans in Poland) was intensely discussed between Kohl and Mazowiecki. This is easily verifiable in any decent historical book about the German reunification. Both sides agreed that any large claims against Germany would be harmful for Polish interests because a) the issue had been largely settled with the treaties of 1953 and 1970 b) it would in return cause Germans expelled from Western Poland seek reparations c) Poland had no way to enforce those claims and no German government could agree to them and survive d) Poland recognized that it would need German support to win access to the EU and that there were huge opportunities for economic cooperation.
@Bobko Thanks for the link, my university library has full digital access to the book, so I might be reading in once I have more time in a few weeks.