Is a store liable if a person doesn’t report a trip?

Not reporting it immediately does not automatically or necessarily mean that the store would not be liable (assuming they would be liable in the first place; see below), but it certainly would weaken that person's case a great deal; it weakens the case because the failure to report immediately makes it look less believable or credible that the accident occured in the store--when someone is injured in a store, they generally report it immediately, so the failure to do so makes it appear that the accident did not occur in the store. It also denies the store the chance to investigate and document the situation effectively (e.g. the clothing may have been picked up and moved before you reported it) and denying them that opportunity also makes it look more likely that the accident in fact occcurred elsewhere and you are simply now trying to blame it on the store to get money from them. So while it does not prevent you from holding the store liable, it certainly  makes it more difficult. 
Also bear in mind that in any event, the store would only be liable if it had been at fault in some way. The fact that clothing was on the floor does not, by itself, mean they were at fault--for example, if the clothing fell only a few moments before you tripped, so that they had no reasonable chance to become aware of it and clean it up, they would not be liable. Liability in a case like this generally depends on the clothing lying there long enough that it would be unreasonable or unbelievable that that the store would not have had the chance to notice the pile of clothing and put it away; in that event, when they had the opportunity to correct things but failed to do so, that failure can make them liable. But if they had no reasonable opportunity to do this, they would not be liable.