Questions regarding the japanese I used

Hello everyone
I got a few questions from when I was visiting Japan.

I was wondering if this type of behavior is kind of normal or if I'm doing something wrong(like talking too slowly)since my japanese level is around JLPT4.
I was asking an train station attendant the following: 駅のスタンプを探しているんですが。but most of the times the attendant would start answering after i say 探してる, so I never really finished the question.
Other example that I can remember is me trying to say ハンバーグを一つとドリンクセットをください。, in this case I would not get to say the last part をください。
I know the important information is at the start of the sentence, but I was wondering if most people would assume you are being polite so it's no use to hear the last part, or maybe was my fault by talking slowly or incorrectly, or simply they were just busy.

The other situation I'm not sure on was on the konbini, when I would be asked if I needed a bag(granted they would ask me something and I would hear ふくろ-something), I would answer with either いいえ、要りません。or いいえ、けっこです。, they would pause a bit before answering or saying thanks, this one I'm not sure if they were asking me something other than what I was thinking.

I have no way of knowing how slow it was that you spoke, but as far as I can see from your post, I see nothing wrong that you said. You asked a question and placed an order in proper, polite Japanese.

Actually, I’d say even if I (Japanese, and I look Japanese alright) asked 駅のスタンプはどこですか, probably the busy station staff would start walking to guide me to the stamp location as soon as he heard 駅のスタンプは from me.

The same would be with your order: as soon as they heard you say ハンバーグを1つとドリンクセットを, they can tell what you want. They do tend to be busy and pretty much matter-of-fact. They usually repeat the order back to you, so that would ensure that they got your order.

About being asked if you need a bag, they probably asked you:　ふくろは　要りますか. Is that what you think you heard? They might have waited to make sure that they got your answer right, or maybe they waited to see if you were going to pack it away into your bag/another shopping bag right away?

Just to add, a great number of konbini clerks are foreign residents (including many non-English-speakers), so they may need an extra second to understand what their customers are saying in Japanese. By the way, I just kind of smile and shake my head to express that I don't need the bag. It's often kind of noisy in there.

As Uco mentioned, if you're at a convenience store, there's a chance the clerk isn't a native Japanese speaker wither. One of the trickiest things to do, linguistically, is communicate with someone in a language that neither of you speak as a native.

In simple terms, imagine, Person A, from the U.S., speaks Japanese well enough that he can communicate 80 percent of the time. So does Person B, who's from Sri Lanka. They both speak the language pretty well, but if their missing 20 percents don't align perfectly (and they won't), they might only be able to understand each other 60 percent of the time.

There's also a possibility that your pronunciation is a little unclear, and it took an extra half-second or so to decipher what you were saying. But いいえ、要りません。 and いいえ、けっこです。 are both perfectly acceptable sentences, so I wouldn't sweat it too much.

Oh I see, thank you everyone for your replies.