I always felt this while watching a David Dhawan movie that you are watching two different films (for the simple reason that he used to copy two films one in the first half and another in the second half) so you will feel that the whole story has been turned upside down without any proper explanation.
So we have Hero no 1 (1997) which was DDLJ in the first half and Bawarchi in the second half. The reason I found the twist unconvincing when the boy turns up at the house of the girl to win them over by acting like bawarchi or cook is because of two reasons- firstly, in the beginning the hero is shown as a lazy man with hardly any significant talent. And I understand people learn a lot of things out of love but when and how he learnt cooking, singing, dancing and all the other skills is never informed to us. secondly, what was his exact strategy- was he going to win over the family as a cook and then expected them to get their daughter married to a cook!! The film anyways doesn't make any sense (the girl goes to Europe for study trip, I wish to know which college sponsor such lavish trip to Switzerland and how come she did nothing academic in the whole trip? )
Similarly, David Dhawan copied the first half of Bade Miyan Chote Miyan from Hollywood film Bad Boys and when they run out of story they causally introduced a twin pair of both the heroes and turned it towards Angoor. Again there was no rationale how come the same set of twins exist at two different places at the same time, we just assume they are doppelgangers. The problem with this genre shifting is that they never care to mould the characters. When the second set of characters most of the characters from the first half are forgotten. So the Ramya Krishnan subplot which was the basis of the first half doesn't come again till the climax.
Thirdly, David Dhawan copied the two biggest hits of 1990 - Dil and Ghayal to make Shola aur Shabnam. The first half of the film is taken from Dil with college romance and light moments. But by the second half the film turns to action mode making a sudden shift in its genre.
This genre shifting i felt unconvincing in another 90s film- Ishq made by Indra Kumar. The entre first half is fun, humour and comedy. But the film shirts its genre completely in the second half that if one matches only the first half and one who matches only the second half may perhaps be watching two different films with same star cast.