Sent emails asking for letters of recommendation, but no responses/agreement to write letter At my undergraduate school I was hired by my advisor to work in his lab. Now I want to apply to graduate school at the same institution and have begun asking for letters of recommendation, but the only responses I've received have been no, I'm unable to write letters at this time or I'm at a meeting (conference) and will reply when I get back (not automated and I'm pretty sure the prof is already back because we work in the same building). 

The other people still haven't responded and it makes me think they're either really busy and can't do it and so are delaying informing me they can't do it or they didn't see it or saw it and forgot about it. The deadline is approaching quickly. 

Is it obnoxious if I email them again to ask if they received it or when I can expect a response?
They are probably just very busy. Try to ask in person if at all possible, since emails just get lost in inboxes.

Also, the referee may not feel they have the time to write a reference, since there can be a substantial upfront cost in time just to figure out what needs to be said. You can help them, and you, by providing a draft of a reference that contains the main things you think should be said. This sounds a bit cheeky, but can be very effective at cutting the perceived effort in writing your reference, and thus improving the chances they'll agree to provide one.