Rúguǒ nǐ méi shōudào tuìkuǎn, jiù zài liánxì yíxià kèfú ba.

没 is used to negate something that did not happen or has not happened, especially in the past or up to now.

So:

• 没收到退款 = didn’t receive the refund / haven’t received the refund

By contrast, 不 is usually used for:

• habits
• general facts
• future refusal/intention
• adjectives

So 不收到退款 would sound unnatural here.

如果 means if and introduces the condition:

• 如果你没收到退款 = if you haven’t received the refund

It is a very common way to mark an if-clause.

It is not always strictly necessary in conversation if the context is already clear, but using it makes the conditional relationship very explicit and natural, especially in standard written or careful spoken Mandarin.

就 often appears in the second part of a conditional sentence after 如果. It marks the result or next step:

• 如果……，就…… = if …, then …

So here:

• 如果你没收到退款，就再联系一下客服吧。
• If you haven’t received the refund, then contact customer service again.

In English, then is often optional, but in Chinese 就 is very common in this pattern.

收到 means to receive successfully / to have received.

The 到 here is a result complement, showing that the action reached its result.

Compare:

• 收 = to receive / accept
• 收到 = to receive and actually get it

So:

• 没收到退款 = didn’t actually receive the refund

Using 收到 makes it clear that the refund has not arrived.

In this sentence, 退款 means refund as a noun:

• 收到退款 = receive the refund

But 退款 can also be used as a verb in other contexts:

• 商家会退款。 = The seller will issue a refund.

So 退款 can function as either a noun or a verb depending on context.

再 means again or once more in this sentence.

So:

• 再联系一下客服 = contact customer service again

It suggests that this is another attempt, probably after a previous contact or after waiting for the refund.

A useful contrast:

• 再 = again, one more time, usually for a future/repeated action
• 又 = again, often for something that has already happened

Here the action is something the person is being told to do next, so 再 is the right choice.

一下 often softens a verb and makes the action sound:

• brief
• casual
• less direct
• more polite

So:

• 联系客服 = contact customer service
• 联系一下客服 = contact customer service for a moment / briefly / just give them a contact

In English we often soften commands with words like just, a bit, or by using a gentler tone. 一下 often serves a similar softening function in Chinese.

This is because of a common tone change rule for 一.

Normally:

• 一 is first tone: yī

But when it comes before a fourth-tone syllable, it changes to second tone:

• 下 is fourth tone: xià
• so 一下 is pronounced yíxià

This is a very common pronunciation rule.

客服 is short for 客户服务, meaning customer service or customer support.

In real usage, 客服 can refer to:

• the customer service department
• a customer service representative
• support staff in general

So 联系一下客服 means contact customer service.

This abbreviation is very common in modern spoken and written Chinese, especially in shopping, apps, and online service contexts.

吧 softens the sentence and makes it sound like a suggestion, recommendation, or gentle instruction rather than a hard command.

Compare:

• 再联系一下客服。 = Contact customer service again.
• 再联系一下客服吧。 = You should contact customer service again / Why don’t you contact customer service again?

So 吧 makes the tone more polite and less forceful.

Chinese often omits a repeated subject when it is already clear.

So this sentence says:

• 如果你没收到退款，就再联系一下客服吧。

A fuller version could be:

• 如果你没收到退款，你就再联系一下客服吧。

Both are grammatical, but the version without the second 你 is more natural and concise.

The sentence follows a very common pattern:

• 如果 + condition， 就 + result/action + 吧

Here that becomes:

• 如果 you haven’t received the refund,
• 就 contact customer service again,
• 吧 softens it into a suggestion

So the structure is:

1. condition: 如果你没收到退款
2. result/action: 就再联系一下客服
3. softening particle: 吧

This is a very useful sentence pattern to learn because it appears often in everyday Mandarin.

Both are possible depending on context, but If you haven’t received the refund is often more natural in English.

That is because 没收到 can refer to:

• something that did not happen in the past, or
• something that has not happened up to now

So in a customer-service situation, the idea is usually:

• If the refund still hasn’t arrived, contact support again.

That is why English often uses the present perfect here.