Long story short: Bale and VDV are the main culprits as they come narrow too early, they need to start wider in order to: 1) Open up space in the middle in the earlier phases of attacks; 2) Make the overlapping runs from the full-back more of a surprise and less of a plan A for width; 3) Give more of an element of surprise and thus a little more space to their own movement; 4) Open up space for the CMs to join the attack.
Ekotto, Bale and Lennon are all very capable at delivering balls from wide positions. Walker, especially in the past few months, has been very poor, I don't think he has come remotely close to picking the right ball, let alone completing a cross. But nonetheless, he is excellent and providing supporting runs outside the player on the right and by this doing this alone he should be having more success.
I think our problems arise elsewhere. When we have Bale thinking he is the new Ronaldo, and VDV vacating his right-wing position right from the start of attacks (in the 4-3-2-1 we've been preferring recently), we have no-one to stretch the play higher up the pitch. This means that there simply is no space for vertical runs from deep. We've seen Modric, Parker and Sandro all make good runs from deep positions in the past, it isn't that they're incapable of doing so just because they prefer to play deeper. The horizontal movement of our wide players has just been awful recently.
As a wide midfielder, even with an attacking full-back to stretch the play on the overlap, you need to start in a wide position. Often, we see Adebayor dropping deep, which is the point at which these wide players should be coming inside and attacking the space he creates, but Bale and VDV have already come inside to see the ball earlier in the move. This is simply selfish and irresponsible. Starting so narrow means that, especially when teams have scored the first goal against us (as in every single game since the Arsenal loss, bar our two wins against Swansea and Bolton), if teams want to suffocate space in the middle, it is very easy for them as everyone they need to pick up is already there.
Normally, the overlapping runs of the full-backs would counter this problem if the wide players were cutting inside regularly. However, when our wider players start so narrow for 80% of moves, the overlap is painfully predictable and easy to defend, it is the only ball that's on, and a bit of pressure and tight positioning in the centre will force the ball out wide to the same player every time. Essentially the overlap isn't an overlap anymore, the full-backs are just auxiliary wide midfielders as they are dragged up the pitch too early, or leave too much space ahead of them, because of the wide players being too narrow.
Finally this brings us to midfield runners. When we do get into advance crossing positions, because the movement of our full-backs is so predictable, even if (like Walker) they make the best out of the situation, they are going to have a difficult job on to get into a good crossing position and when they do manage a good cross, the centre is so congested that bombing forward will offer little to the attack as there is not enough space to control the ball and get a shot off, or to shoot first time. If the wide players' starting positions kept the pitch a bit more open, we may see more of the dribbles/runs deep in the opposition half that Modric used to make multiple times a game (eg. for his goal vs Liverpool in the 4-0), while runs inside/overlaps may be a little less predictable, helping to open up the half yard for the shots that VDV and Defoe especially love to hit from around the edge of the area.
Bale and VDV want to see too much of the ball to play in the positions they currently play in. Sure, they should come inside, and sure they can start some moves centrally, but the majority of the time at least one of them, if not both has to be 0-5 yards from the touchline when we get the ball. Barca occasionally struggle from a lack of natural width, but they don't need it to open up space, because players like Xavi, Iniesta and Messi open up this space with their incredible passing, control and movement. For them, congesting the middle is beneficial as they can control the game due to their massive superiority over nearly every team they face and their immense ability on the ball (funnily enough, this wasn't enough against Chelsea in either leg, which is comparable to Spurs playing most of the other teams in the league when they decide to set out as Chelsea did against Barca).