Help - Losing Ranking on Plural Keywords in Google

[BeckyKey]

Help!

I'm losing keyword rankings for certain keywords - but still ranking well on others.

The trend I can see is that we're losing rankings on plurals, so this page https://www.key.co.uk/en/key/platform-trolleys-trucks

Used to rank well for:

platform trolleys
platform trucks
flat bed trolleys

But now only ranks well for

platform trolley

flat bed trolleys

platform truck

I have seen a large number of keywords drop out - they're not hitting traffic massively yet, but conversions are dropping - help!

I;m struggling to find the cause!