Bicycle Mechanics - chain slipping?

less than 1000 miles. I clean the chain from time to time, but am not obsessive.
Is this likely a worn chain, or should I be looking for other causes?
Advice?
thanks

Too little info, have you had a new cassette recently and not changed the chain, it is the RD which needs some adjustment? what do you mean by slipping?

Worn chain and/or worn rear cassette/freewheel gears and/or worn front chainrings are the common causes.

The various causes of chain slippage manifest differently.

Worn chains or sprockets generally slip or skip under high load, and seem selective, slipping on only certain sprockets or chainrings.

If the slippage is more randomized, and occurs at high pedal cadence or when not under load, then it's probably not a wear issue. But start by measuring 24 links (12") of chain under light tension. If the theoretical 12" of chain measure more than 12-1/8" (pin-to-pin on centers, or any identical reference point), then wear is the likely cause. At 12-1/16" the chain usually wouldn't skip, but you should replace it to prevent rapid cassette wear.

The other causes of slippage, include RD trim, damaged sprocket teeth, stiff or damaged chain links (more common on mtb than road), ghost shifting from cable issues.

If you're comfortable that it doesn't seem to be a wear issue, and it shouldn't be at only 1,000 miles unless they were very hard miles with a poorly lubed chain, then you need to work through the list of non-wear possibilities.

The chain was installed earlier this year, the cassettes have about 4 k miles on them, it slips under load, and sometimes I find i cannot change gear untill i push the lever twice. I really do not want to replace everything if it is a simple fix.

thanks

You omitted a key point in your OP. While you said the chain was relatively new, you failed to mention that it was running on an older cassette. Odds are you have a classic new chain/old cassette issue. Normally these show up immediately when the chain is replaced, but yours might have been on the edge and need a bit more time.

Start by measuring tour chain for stretch so you know where you stand, then see if you have or can borrow a comparable cassette wheel in newer condition. If your chain runs the other cassette OK, then yours is toast.

If replacing the cassette, you face a dilemma, since your worn chain will age it more rapidly than a brand new chain would. In your shoes, I'd replace both the chain can cassette, and keep your existing chain to rotate in after 1,000 miles when the old chain catches up in wear. Then switch back and forth at set intervals to keep both chains and the cassette matched in wear.

This is the best way to maximize life of both parts and enjoy smoother running longer than sequential chain replacement at 1/2% stretch. I keep 3 chains per bike, rotating them at about 1,000 miles, and by doing so am normally able to run them all well past the normal replacement point, some times 3-4 times beyond (1.5-2% stretch), though obviously by then the cassette is toast.

To make rotation easier, each chain has a reusable connector link. And while on deck the chains are lubed and prepared for remounting.