Unfortunately cellular radios are not always the best at finding service through
certain materials(like that of buildings) and sometimes after reaching a certain depth. For example there was a time when Sprint users would complain because sometimes they couldn't use their cell phones in their own houses, only outside. The same happened with verizon for a while. Coverage as far as area is one thing, coverage WITHIN a structure probably says in the contract that the company can't be liable because of the wide range of variability. Chances are your desk or office or work area, is just within that blocked area and the signal can't make it through. For whatever its worth to you, whenever I would enter the basement or the stadium of a former workplace, once I was inside all that concrete, all my phones dropped signal and would roam and search for hours, wasting my battery.
Roaming by all intents and purposes is designed to find a signal when your regular straight talk signal is lost - in many cases(as shown by some companies upcharges) it involves using the tower of another carrier. BUT it says specifically on the straight talk website that the their phones do NOT roam, which is strange but understandable if they have specific boundaries on their phones. So your phone may try
to roam, but because the signal is lost, its simply not finding anything.
The other possibility would be if something happened to one of the parts inside and it limits the reception of the phone.
Finally, just so you have it Customer service for Straight talk is 1-877-430-CELL (2355) and on the straight talk website, they mention wanting their customers to call IF their phones roam, because they are not supposed to. Also if you click here
you will find the FAQ and the website for straight talk.