Hi - Only think I can think of, assuming you have not changed anything about the original campaign and that is not market forces related is if you are using a Shared budget as opposed to individual budgets?
Other than that could be worth checking market forces by which I mean:
- have competitors raised their bids
- is it seasonal i.e. you got higher impressions pre-Christmas
- have competitors lowered their prices relative to yours
Do you see any changes in your competitor metrics e.g. Search Lost IS (Rank) etc.? Has your Ad Rank changed?
Have you tried doing a sandwich test? i.e. one product only for a few days, multiple products for a few days, back to one product for a few days (use same days of week if you can). This would help you iron out if it's truly one vs multiple products that is causing the issue.
Not sure if we are following correctly but in your example of 3-4 products in one campaign:
If this is the case, then its simply a matter of having multiple products in one campaign to which they would all share the same budget and ads would be displayed based on whatever search terms were used by the shopper on Google.
This is completely natural.
Could you clarify a bit more?